Based on what has been publicly displayed of JJ2.5, there's very little to discuss. It's a level viewer, and moreover a level viewer that has only been shown on three very graphically simple levels, making it nigh-impossible to judge the accuracy of the layer rendering. Big deal. J2O has a level viewer too, as stijn mentioned. I've written level viewing code in C# and also Python of all things. DJazz has written level viewing code multiple times over, including 100% accurate textured backgrounds. Without a lot more information there's not much to say, and more importantly, it has bugger all to do with this thread.

So! minmay. We don't know. There are 512 blank bytes in a standard .j2l file following the animated tiles section, which would hold an additional three tiles. Any more than that would require adding extra bytes to data1, which we know is in principle possible because that's how one uses text strings higher than #15 in 1.23 levels. However, doing that is somewhat unstable and we've historically left it alone because a) the technique doesn't work in TSF for whatever reason and b) loading all those extra bytes seems likely to have unknown and unpredictable effects on the game. If you make data1 too long, for example, you erase all control data. There may be a middle ground, but it's largely unexplored terrain. And all that's assuming that JJ2 would even accept animated tile numbers higher than 128 in the first place, which I haven't checked.

Perhaps a better reply would be: what are you trying to do? Maybe there's a better way to achieve it than the one you're thinking about?