Kate, a lot of institutions still use permanent marking systems. http://www.gaylord.com/adblock.asp?abid=16720&search_by=desc&search_for=collections%20labeling&mpc=WW
might give you some ideas for the bottoms or the pieces. I think for temporary marking for exhibits, using a Stabilo pencil on the bottom (if it’s glazed) would work, but if there’s no reason not to, I’d use the more permanent ink and barrier coats. I have a lot of weirdly shaped plastic things in our collection (can’t use those marking systems on plastic), and I use cotton tape with a label and then just wrap it around however it will stay – looks a lot like wrapping Christmas presents. That’s only useful for storage, though.
Bird poop is usually removable with water, the more resistance stuff can be removed with vinegar, but i’ll leave the appropriateness of that to the conservation people.