Isn't this really only a problem when you waive everyone on your 40-man roster? If you just put one or two guys on there, it's fairly easy to remember. And becomes even easier when you just send out a trade chat asking the other owner what they'd be willing to give up. Maybe we should go to the MLB rule where you can only have 7 guys on revocable waivers at a time. That would make it easier to track.
But I suppose if you too lazy or irresponsible to trade chat, or post in player's notes, or not delete the message, maybe have on icon on the GM page like with Rule 5 draft. The when you rollover it could tell you "Cleared" or "Claimed by XXX".