I suggest you check Johs Putnam's list here:
http://www.phred.org/~josh/build/brazeons.html

As for your questions:

(1) Can old Campy down-tube shifters be converted from clamp-on to braze-on?

Yes, if you find the proper braze ons. Or would it be the occasion to upgrade to indexed shifters? Or to move them to bar-end shifters and use the downtube stops for the Shimano screw-on barrel adjusters?

Add an eyelet near the headtube, through which the cable housing will go, so it won't rub the paint.

(2) Can I get a braze-on front derailleur for a wide-range triple (24-40-54)?

Yes, but many don't recommend it. Rivendell states clearly on their website that they have seen frames breaking at that point and I have read about some manufacturers strenghtening their fraime (i.e. thicker walls) to account for the braze on. I have also read about people with broken frame. Besides, once you position the braze on, there is no coming back. What if you finally prefer 24-38-48 and need to slide the derailleur down by 5-6 mm?

(3)Other ideas?
If your frame is large, can you position two bottles along the downtube, plus one on the seattube and one under the downtube?
Pump peg. My preferred pump is the Topeak Road Morph, with bracket installed under a bottle
Brakes. I am assuming you have a road bike. Are the brakes good? Are you considering a replacement to cantilever or v-brakes? If so, canti posts might be good.