I commission a fantasy football league with pretty much homebrew rules. The relevant one for this discussion is this: "Only players that have used up or relinquished their college eligibility may be on a roster at any time." If you read the subject of this post, you can see where this is headed. Someone drafted Mike Williams with the 13th pick (it's a large dynasty league -- 160 skill position players on rosters, with about 120 protected from 2003) with the intention of keeping him on the bench for a year. I allowed this, with the stipulation that if his NCAA eligibility was reinstated, the drafting team would lose him. What would you have done?
So, what you are saying is that you run a keeper league? Hmm, that is pretty interesting. I would probably disallow it, as I think it is silly to have a guy who is not EVEN in the league on a team for a whole year. For the record, I think Williams will be a great NFL player but I doubt he will be that good in his rookie year, especially after having a year off. To get around that loophole, maybe rewrite your rules of eligibility to state any player who is with an NFL team instead of "Only players that have used up or relinquished their college eligibility may be on a roster at any time."
I would allow it, and then create a rule and name it after the dork who did it. The "on an nfl" team idea would work. My league has "The Pizza Hut Rule" when I traded Brad Daugherty for two slizes of Pizza. He had Virtigo or wahtever so it seemed like a good deal.