Coffee beans, even ground up, are no more likely to significantly change a soils pH then would Oak leaves and pine needles. Earthworms are reported to really like coffee grounds but earthworms do not like a soil that is too acidic,

Hi!

"My potting soil is pretty neutral, and I'd like to bring it down to ~5.5. "

Neutral... what tells you this - are you using one of those ph monitors found in a garden store? Is this potting soil something you buy and place in containers?

Freshly ground coffee seems like an awfully expensive amendment. Tell us more about your garden and your pH testing, and have you grown these peppers before?

"beaten to death". If you read all the post, then you know I am pro coffee waste. But I do not thing it will help lower your pH.
You need sulfur, A little at a time, until you have what you want. I put lime around my "cow horn" hot peppers last year & gave all I did not pickle away to 3 families.
I lime my beds every year, because I plant 10 to 12 months a year on my beds. The cole family are heavy feeders.

Thanks for the responses. :)

Yes, I'm using a ph meter I have for measuring soil. It says it's 6.8. pretty close to neutral. I don't know how accurate it is, as long as it's close. So maybe give or take a few points.

I have grown peppers before, and I didn't do anything to the soil. So it's mostly an experiment on my part just to see how they would do if the soil was the "right" ph. So it's not really necessary, I suppose, but I'm just curious.

Plus I have some rather exotic varieties this year, and I'd like to give them the best chance at fruiting that I can.

Many potting soils are primarily peat moss with some peerlite, or vermiculite, added to aid drainage. As long as you provided a nutrient source plants have grown quite well in those potting soils without adjusting the pH.
To see if your pH meter is reading anything use it to test some vinegar or lemon juice and a baking soda solution.

"Right" pH is a bit of a misnomer -- most plants will tolerate, even thrive, in a relatively wide range of pH and peppers are no exception. A quick Google search turned up the following ranges: 5.5-7.0, 6.0-6.5, 6.5-7.5. Unless you were growing a specifically acid-loving plant (blueberries, potatoes, etc.), a slightly acidic soil (6.3-6.8) is the preferred range for the largest number of plants.

I wouldn't bother to do any adjusting to a potting soil. The need for frequent supplemental fertilization for any containerized plant will override any need for fine tuning the pH any further.

I wouldn't worry about your pH. Mine is off the charts high, like over 9.0 and I also grow my peppers near concrete so it's even higher. My hot peppers thrive in AZ. I've had some live to be two years old.
They would have lived longer if we didn't have several days in a row of 20 degree temps last winter.