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!