I've replaced most of my stainless steel or chrome bolts with black oxide ones (all for aesthetic reasons). Over time they begin to rust. Any suggestions on a coating that would prevent this?

this will happen no matter what you do.
My way of keeping the bike looking good is to buy an all purpose heat black spray paint and respray them myself when they look shabby. saves money too

Several people on another Forum recommended a product called Boeshield. Developed by Boeing for aircraft components.

http://www.boeshield.com/