"BTW, is it ok (or not) to run the low voltage cord through the wall?"

Absolutely not.

The usual method is to cut an exact size opening for the new box.

You can then run a fish down to the old box.

If the old box has internal clamps you can remove another knockout from the inside after loosening (and sometimes even removing) the clamp.

A new cable is fed in from the old box and pulled up to the location of the new box.

An 'old work' box (they have clipped corners) is then used for the new outlet.

The cable is fed into a knockout in the old work box, and the box installed in the opening.

Either drywall wings or Madison clamps can be used to anchor the new box.

The most common problem is fire blocking in the wall cavity between the old lower receptacle outlet box, and the new box location.
You can use flex bits to drill through the fire stop, or open the wall again to dill through the fire stop.

Sometimes a larger hole (stud to stud and box to box) is easier to wire and then repair than trying to minimize the wall damage.

Drywall is very easy to patch, and plaster only a little worse.

Thanks. Drywall repair is easy, but I think this can be done without it. Also, no fire stops in this interior wall.

Regarding the metal box, the only available knockouts face downward, besides an oversized one in the center of the back wall. Still, it seems too tight for another wire pair, pigtails + receptacle in there, which is why I'm tempted to install a larger box next to it, and use that box for the HDMI cable.

Can an HDMI cable be run through the wall? That's not considered low voltage, or is it?

I now understand that the HDMI would have to be "in-wall rated" to put it in the wall.

It wasn't clear to me from your description, but you can't run cable/telephone/tv or the like in the same box as you run your power. You can use a separate "normal" box, or they make low-voltage boxes, "cut-in" or "old-work" is what you need, often orange in color and without backs, so that you can trim the hole in the wall with a faceplate.

If there are already two romex cables and a device it the existing box, you are going to be pushing it on box fill limits. It may be easier all around to just remove the existing outlet box (cut nails with sawzall or pry off of stud). Enlarge the hole to accept a 2 gang old work box, this will give you more working room and allow for more box fill capacity. After fishing your new wire to your new TV receptacle, install the two gang old work box.
Good luck.

What NEC rule says that you can't run an HDMI cable inside a wall? I understand that there may be additional rules if the wall serves as a plenum, but otherwise am having trouble finding a rule that prohibits the installation as described by the OP.

"What NEC rule says that you can't run an HDMI cable inside a wall?"

None.

The only issue is plenum rated vs. non-plenum rated.

Walls as plenums show up sometimes in residential, but not very often.

Elsewhere I noticed several folks say that CL-2 rating is minimum for in-wall use. Also, here's a gardenweb thread on the topic. There's also a link in the thread regarding the codes for this stuff.

http://ths.gardenweb.com/forums/load/wiring/msg0917215626722.html

I should have just posted the other link regarding the code that may apply. Here it is:

http://www.audioholics.com/education/cables/understanding-in-wall-speaker-video-and-audio-cable-ratings

I did a similar thing in my home, if you eliminate the device in the existing box then you can splice there and pull up to your new outlet probably without violating box fill, you will still have a receptacle in that horizontal spacing, albeit behind the TV so code should be OK ;). You can buy, and I recommend a combo recessed outlet/surge suppressor with rated LV side, that cuts in at about 1 and 1/2 gang old work. pull your video, etc up in and through the LV side, but since you have 12/2 in the old box, it's probably on a 20A so pull 12ga up to this new box as well.

I just painted a blank to match my wall and covered the old outlet box, and it just disappears visaully, cause everyone's looking at the TV anyway.

KEN

Typically the new recessed boxes for behind the tv offer a dividing plate that make it ok/legal (at least here in BC) for power to co-exist in the box with low voltage - ie one section/gang is power, separated. Next section/s will be your RG6 cable, cat 5e/6 for internet, etc.

As for running the hdmi through the wall, legalities/plenum rating aside - since it's most likely 'temporary' since it feeds in and out of the wall, as a pre-made cable, it may not be much of an issue.

What we do in new construction is run a piece of 'core flex' flexible conduit in the wall, rated for such use, from the tv box down to the spot where you'll have your blu-ray, cable box etc, together with a pull string - you can feed your cable through it and theoretically it's no longer 'in the wall' since it's enclosed. I'm not going into the legalities.

You can also use central vac pipe - if you made the opening for it just an LV mud ring, you might even be able to feed the stuff into an existing wall. It runs roughly between the tv box and the box for the cable etc, again as a guide for the hdmi.

For a nice looking finish, you can either use fittings that are a coupler for cables, so your hdmi would plug into this coupler/finish plate, and you'd use another cable to run to tv/cable etc.

That ends up being a lot of cables and connections, what's simpler is a "nose plate" that fits a decora plate and is a hooded arrangement you can have sticking out, or recessed in, and you pull your hdmi or whatever cable out, so you only need a single.

This also allows you to bring a few others out as well, component, audio etc. I've thrown in one example, I'm sure there's a bunch of manufacturers and suppliers.

Here is a link that might be useful: decora nose plate

Since a typical stud cavity is not a plenum, and you are not going floor to floor to even consider it a riser, the rules do not apply.

"I should have just posted the other link regarding the code that may apply. Here it is: "

"may" apply???

It does NOT.

You are very unlikely to have risers or plenums in stud walls in residential use.

Occasionally someone will 'pan' a joist cavity or stud cavity to use as a plenum, but it is far from common.