not to stir up a blog controversy (actually, we could use one of those) but i think that the answer is no, you can't trade the 'idea' of a keeper slot. some answers to your likely counter arguments, and an alternative below.
1) we already expanded the number of keeper slots once - i would say that we give this whole thing a shot as written before we tinker with it again, because no one wants to realize that keepers turned 4 teams into YOUR kc royals and have to restart everything/talk bench off of a highway overpass.
2) true, we can trade draft picks. i think the difference is this: trading draft picks isn't just for kicks - it gives people who drew a draft slot they don't like (the sandwich pick, or whatever) a chance to take fate into their own hands and trade for a pick that is more their liking. no parallel justification for keeper trading.
3) also, for draft picks, the 'expected value' (work with me here) of a draft pick is ALWAYS going to be positive (a draft pick is always something you would want, pretty much no matter what). this checks abuses on the system - no one would do something so rash as trade their last pick for a coffee because you might draft the next mark teahen (billy butler, for those of you who were wondering. he's actually the next ellis burks, but like i said, work with me). not the case for keepers: let's say, for instance, you drafted a thousand pitchers lsat year and you are sizing up your roster and realize that you have no position players 'worth' keeping. ('worth keeping' meaning that their keeper value exceeds their expected draft number). so in our 'market' of keeper slots, depending on the owner, some keeper slots could have a negative/zero value to the owner. pretty easy for bad things to happen - you might have people dumping keeper slots to the huge advantage of another owner who is in better position in the draft because his rent-a-closers improbably finished at the top of the league once again.
4) what you COULD do is trade your dudes straight up, so that pete or whomever was sitting on the right to 'keep' the player in question, and then re-acquire those players for papelbon after the draft happened. the potential for backstabbing making this just all the more exciting.
no one trades anything in our league, so i guess this is all kind of academic, but i don't think it's a good idea.
i just want to say that i was a colts field goal on that last drive away from $93 4-way prop bet. fuck the colts.