The following requires the help of a friend at the USPS, or going to a Notary Public. I made the following up years ago for touring overseas, and for crossing the US Canadian border with my bike.

Photo the bike small enough to fit within a credit card sized print. Write the serial number on the back, along with your ID info and the date. Then bring to the post office and ask the clerk to post mark it for you (or have a notary sign and date). Laminate the whole thing and keep with you when touring, especially when crossing borders. To make it more conclusive, have a friend photograph you with the bike, and maybe include some recognizable landmark or newspaper headline to establish age and location- this is overkill, but why not?.

In the many times I've reentered the US and been asked to prove prior ownership of the bike, and that has always been accepted without challenge.

In this age of PDAs and cellphones you could probably update this concept with a photo showing you and the bike in front of something that can establish location.