I was looking for somewhere to post this anyway, since it doesn't seem to be mentioned anywhere.
How to edit raws for forgotten beasts (and hopefully other generated creature definitions):
Make sure compressed saves are OFF in init/init.txt (obviously).
Open world.sav in a hex editor and search for the creature description.
The creature raws will be stored as a 32-bit COUNT, followed by COUNT number of LENGTH+STRING entries, where each entry is a 16-bit LENGTH, followed by LENGTH number of bytes, then the next entry. COUNTs and LENGTHs are both little-endian. Eg "A3 00 5B 44 45 53 ..." for a 163-byte DESCRIPTION tag. DESCRIPTION is likely to be the last tag, followed by the COUNT for the next creature (realisically, some nonzero byte followed by 3 zeroes, while LENGTHs are a nonzero byte followed by one zero). The first entry after the COUNT will be 000E "creature_layer" (or whatever's appropriate to your non-forgotten-beast definition).
If you change a entry, you need to update its LENGTH field to match the new number of bytes. No other changes seem to be needed. Eg, adding "did this work?" to the above DESCRIPTION would require 00A3 + 000E = 00B1 ("B1 00 5B 44 45 53 ...").
If you add or remove entries, you need to update COUNT plus or minus the net change in number of entries. You will also need to make sure any new entries have correct LENGTH fields. Again, no other changes seem to be needed; there's no checksums and the total length in bytes of generated raws doesn't seem to be stored.