I'm using a tag to let wishers know there's a copy w/ diff ISBN in the system. Here's an example:

Link

I put the tag on the ISBN that wishers are wishing for letting them know when they look at it that there's a similar book available. I also put the ISBN of my book in the tag so they know what to look for.

My book has since been requested, so I changed the ISBN on the tag to zeroes so you could see how I did it.

This is a great idea - as long as everyone remembers to delete their tag afterwards!! 

It's like remembering to delete a post in the BB, except sooner or later you're going to see it in your tags and and delete it.