I did a comprehensive fluid change/bleed on the brakes yesterday and have 2 calipers, one rear one front, where only one of the bleed nipples seem to work (one upper, one lower). Brakes are working fine with good pedal feel. I'm curious what might cause this?

Dirt, crap, rust packed in the bleeder.
Here we have the mud dauber wasp that will clog any uncovered small orifice.


https://www.youtube.com/watch?v=knLthxuxQ8M

If anyone had switched to DOT 5 at any time, you could have what we call "Caliper Worms". The mixture of DOT3-4 with DOT 5 causes a paste or caulk-like substance to build up in the fluid galleys. This will then block the bleeder valve area.

When they are forced out behind the pistons, they look like little worms in the piston bores, hence, "Caliper Worms".

Might removing the stopped-up bleeder and pushing the pedal blow out any debris, since the smallest orifice would be where the barely-open bleeder seats in the caliper?

Does "good pedal feel" = working caliper?

Yes. Could be clogged bleed screws, but both? Remove one at a time, inspect and observe if BF dribbles out of the caliper (pedal stroke not necessary). Just make sure fluid in reservoir doesn't empty. A golf tee might be handy for a temp. plug.

If bench testing, you could observe if pistons move with compressed air (Careful. Block excessive piston travel). If installed on the rear, you may not feel one non-op caliper during normal driving.

Caliper not working? Check fluid flow through the flex line to the caliper. The inner passage on old lines could be blocked by age. This results in normal pedal feel, but a non-op caliper. Something to check.

Sherwood

The calipers are working. The bleeder orifices appear to be blocked. Without taking the calipers off I can't tell any more than that (and I don't think I have worms... ew!). I guess what I really want to know is if this means the calipers aren't functioning correctly or can I live this way for awhile until I can send them to PMB for refurb?

Sure... if it "is" the mixture of DOT 3-4 with DOT 5, the entire system would be affected.

I agree, take the bleeder completely out and see if it's just the bleeder itself or a plugged fluid galley.

Drive it. Does it pull to one side or the other? HF makes a inexpensive but highly operational laser temp gun. Are all of the calipers (pair by pair) within a standard temp range?