Bob A wrote ...

1 1/2 cup sugar in water, not quite hot enough to be painful to the hand.
Not quite a teaspoon of yeast. Gently shaking until all sugar was
dissolved. This is in a 2L soda bottle. I put on a cap tightly and after
about 1.5 hours I could tell there was significant pressure in the bottle,
so it was producing.

Formula sounds good, but the water only needed to be lukewarm. If the
yeasties lived the surface of the water should have a little tan foam on it.

I now have the bottle hooked to airline hose (silicone needs to be
ordered,
using regular airline hose for now) running through a gang valve with
check
valve, and into the intake of my power filter.

I use regular aquarium tubing. The most common problem I have is losing gas
at gang valves and check valves! I've gotten rid of as many as I could. If
you have the bottle on top of the tank anyway, I'd recommend attaching the
hose directly from the bottle to powerhead intake. Caution: if you do this
while the liquid is still warm it may partially collapse your soda bottle.
A juice bottle is sturdier.

This morning the pH is neither down nor up. Still about 7.3. I'm
guessing
(or think I've read) that I might need to use some baker's yeast too.

I don't think so. Bakers yeast is faster (that's all I use because I bake
bread, too) but, at this point, you should be producing. That's the big
draw back with using the powerhead, is that you can't see the bubbles (oh,
and collapsing soda bottles sometimes). I'd be more inclined to suspect
either a) leaking fittings, or b) terminally-cooked yeasties.

...I have the bottle sitting on a vent from the light, to warm it up some.

Good idea. Good luck.