If you feel very creative (I mean very, very creative) you could figure out the compatibility matrix and setup ES tribe node accordingly. I have no idea if this could work as there are all sorts of breaking changes between 2.x and 6.x index and mapping wise but it could be fun to test it.
So you could end up with your 2.x <- max compat version 4 <- max compat version 5 <- version 6. (You might need some in between steps).
Again, just something I thought about while reading your post. If it doesn't make sense please ignore it.
Paul.