Well, you might have to wait until Infinity comes out for an in situ editor, or one that can handle sprites, but this nifty little tool should help immensely in the meantime. It allows you to extract wall textures from M2 Shapes files, and save them in M1 format. You can then edit them with any of the Marathon 1 Shapes editors, and then reinsert them. I only did preliminary testing, and there are quite a few restrictions (read the docs), but it seems to do exactly what it say it'll do. Texture editors, go at it!

This is a really cool little tool of use to both mapmakers and players. Now works with both M1 and M2. Drop a map on it, and it'll create a text file containing lots of information about the map. Object info, lighting types, textureset, number of polys, number of platforms, number of points, total volume, and on and on... Use it for error checking (your map is crashing and you don't know why? Make sure the landscape texture you used is the one defined for the map. Want to make sure you haven't passed the 64 platform limit in your map? Save yourself lots of tedious counting with this. yadda...