You should probably check out the Rod and Reel Repair forum, and post your question there.
I know there are guys who have successfully painted reels who posted their results there.
Correction: I just checked that forum and couldn't find anything about how to top coat reels after painting them. Just a few people asking the same question you asked.
On another forum, somewhere, someone used an auto clear, but it was a sovent based and toxic. I'm pretty sure that's the only finish that will hold up.