Not that I expect anyone cares particularly, but I thought I'd bring word of the decision reached in our league in case other commissioners encounter this issue and are wondering how to handle it.
My two League Presidents were initially split on the issue. After presenting both sides, the LP supporting the creation of a house rule was convinced that allowing the market to play itself out was an acceptable solution (Raidergoo, your arguments played in quite a bit here). Based on their consensus, I'm going to forego creating a house rule to handle multi-year in-season contracts.
I won't lie: I still very much dislike these contracts and wish the AI was smart enough to prevent players from accepting them. However, doing the right thing for the league is more important than satisfying my personal preferences. Of course there's going to be a contingent of owners who aren't going to be particularly pleased with my "solution", but I guess that's part and parcel with the whole commissioner thing.
So there you go: though it doesn't get nearly the traffic as the others, this forum does indeed work! Hey, if you make a difference with even one league, it's all worth it, right?