Instead of trying to program something too special to show abandoned accounts, I was wondering if there could be a special server Guild that has no expiration and can take unlimited members. It could be simply called "Abandoned Account" or something like that.
Then, any account that is formally abandoned (you ask for admittance into the Guild) or abandoned by time (4 week non-activity = auto-inclusion) would be added to that Guild.

We would also then not presumably have them show up on Friend recommendations, etc...