I am experiencing a situation where the chain rubs on the bottom of the FD (underside of it) when on the large ring and 29t rear cog. It is not chain rub on either side of the FD, as raising the FD up eliminates the issue and I can see it is rubbing on the underside of the FD.

When set up so that there is no rub on the 29 cog (vertically and internally) the FD is up so high the front ring shifting is unreliable (chain drops far too often when used without gingerly dealing with this issue while shifting). I can get an acceptable performance on all but the largest (29t) cog.

This is a braze on FD, and the hanger on the bike is perfectly aligned. All parts are campy chorus. Seeking suggestions of how to deal with the FD I have, or seeking input on a FD that allows the 29t to work without this annoying rub and keeps the chain reasonably well put when shifting between rings.

Trim the left shifter till it clears , do it after the rear shift too ..

I think OP means that in big-big, the chain rubs the underside of the "crossbar" of the front derailleur, that spans the 2 sides of the FD cage.
And raising the derailleur to prevent rubbing, results in poor shifting.

Can you post a side view picture of the FD when in big-big ?

Don't ride in the big-big combination.

I spent last evening tweaking the height of the derailleur and have it about optimized (given the tradeoff between height above the rings and probability of chain drop). It is up higher than the mfr suggested. I had several small to big ring shifts in today's ride, in relative anger, with no chain drops. The FD still rubs on the 29 cog, but in a less annoying fashion. The rub is from the chain riding the top of the chainring until settling in (a product of the cross chaining). It rubs more "off power" than "on" so maybe this will improve as the rings wear in a little. At the mfr height, it rubbed a little on the next smaller cog, and it doesn't do that anymore as it's set now.

I would like a FD with more clearance, so I may try an old rival unit that I have around. But if things stay as it is, I may not mess with it further.

Wait, you are in the 29 and small ring, then shift to the big ring ?!?!?
That's rubbing the cage while the chain is trying to shift to the big ring ??
Don't do that, shift the rear out of the 29 a few gears (25, 23 ...). Then shift to the big ring if you need to when you have more speed, and there will be less load on the FD and chain.

When shifting to the big ring, I am generally on the middle of the cassette. The rub is just in the big/big combo. The issue was that in raising the FD until the rub was eliminated, the chain would dump off the outside of the big ring (when shifted in the middle of the cassette as normal). A reasonably happy medium was found, but still not happy with chain rub (even big-big) on a four figure modern group.