They did attempt something like that back with the 2.0.x branch, but considering how that went and the scope of such a thing, I doubt it will be considered again.
Many of the security improvements to the new versions of WordPress in the last couple of years were complete reworks of how various systems were handled. Porting those changes to the 2.0.x branch would have been a monumental task and could have introduced instability or new bugs. . . I’m disappointed that we weren’t able to keep the branch maintained until 2010, but since one of the big reasons for that failure was the massive scope of our security improvements for the newer versions of WordPress, 2.0.x doesn’t die in vain!
I'd imagine that the volunteer developers would encounter the same issues while working on an LTS branch of 2.9.x or 3.0.x.