Aluminum is very narrow but will likely not be Energy Star.

Fiberglass is probably your next thinnest frame available.

I am also in the SF Bay area and our house used to have the same style steel frame casements.

You are correct in worrying about the frame size around the glass. Aluminum is strongest, so has the narrowest frames. But on the casements, the frames will be larger than the old steel ones just due to the need to support the additional weight of two panes rather than one.

For various reasons I ended up using Milgard aluminum with low-E glass. We like the looks and how they work. But there is no question that the frame portion conducts heat and cold. Mitigating this is that because the frames are thin, there is more glass than frame area - compared to other frame styles.

Some manufacturers offer a 'thermal break' aluminum frame design. Milgard has them too, but for some unknown reason, does not offer them in California. (Anyone know why?)

Hopes, Torrence, Optimum and Bliss Nor all make a Steel Framed Casement that will accept insulated glass. While not the most energy efficient units they will give you the site lines your looking for.

Thanks for the pointers. I have checked specs for Torrence, Optimum, Bliss Nor, and now Milgard Aluminum. All of them have ~2 1/4" frame for hinged side of a dual-pane casement. Even the steel options don't get close to the 1" frame that I currently have. (Current windows have somewhat unsightly hinges, so there is some improvement there.)

Looks like, as wws944 mentions, dual pane means there is no way to be as thin as before. Part of me wonders why some of the "chunky" frame can't somehow be hidden behind drywall and siding to give the same overall effect.

At any rate, I am going to find a dealer and get a close look at those Milgard Aluminum casements. A little heat/cold transfer isn't a huge deal in our mild climate. Will still be much better than current single-pane windows.

Any other thoughts or advice most appreciated.

You also have some local manufacturers of aluminum casements that you should look at: Bonelli and Ventana Aluminum in San Francisco; Paramount Aluminum in Oakland and All Weather Aluminum in Vacaville. To achieve the look that you are seeking, ask for information on their one inch frames (if they still have them). The aluminum market in the bay area collapsed in the early 90's with Title 24 requirements. The manufacturers that I mentioned are still in business but doing mainly commercial work. Bonelli is the only one that sells direct as the other ones use dealers like glass shops and lumber yards (much like Milgard). Good luck!