If I ever decide to upgrade to MT 3.0, it looks like someone has made the MTThreadedComments plugin workwith MT 3.0.
I could probably have figured it out myself, but it's nice to know that someone has already put forth the effort.
If I do decide to upgrade to MT 3, I might look into the new plugin API and see if this can be made into a more supportable hack.
Update: There's another patch (for 3.11) here.