Yeah, I actually made my most recent .mw2 file in a hex editor. I use a simple pattern for all sprites:
00 (start of the file)
[89 70 xx 8D 70 xx]n, where xx is the sprite number and n is the total number of sprites inserted
FF (end of the file)
The second byte of each group of three bytes is always 70, the third byte is the sprite number, and the first byte varies depending on the extra bits. For extra bit setting 0, it would be 81; for extra bit setting 1, it would be 85; for extra bit setting 2, it would be 89; and for extra bit setting 3, it would be 8D.
So, if I were inserting sprites 00-05, the raw hex data would look like this:
00 89 70 00 8D 70 00 89 70 01 8D 70 01 89 70 02 8D 70 02 89 70 03 8D 70 03 89 70 04 8D 70 04 89 70 05 8D 70 05 FF
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I'm working on a hack! Check it out
here. Progress: 64/95 levels.