I'd say none of the hurdles involved would be too huge, but there are a decent number of them. You'd need an alternative to .j2t files that stored things in 16-bit color instead of 8-bit, and each one would need to include a 256-color palette for sprites to use. JJ2+ would need to be able to read those alternative files and disable 8-bit mode when one was in use. And you'd need some software capable of editing levels that used those files.

Of course, you'd also want anti-aliasing, which might take a more major rewrite.