Wow, thanks for all that information Zoinkity
! I hadn't even thought of using a tree to search. Hopefully I'll be able to wrap my head around all of it.
Update: After a couple missteps, I was able to make a functioning compression/insertion routine:
(The whole screen excluding the text-box is compressed, but for my purposes the only thing that's important is the top left corner, haha)
Oddly enough, my algorithm seems to create longer runs of compressed data compared to the original - I've decompressed then recompressed several game images and they're all at least 100 - 200 bytes smaller. I assume the original Wonder Swan team's algorithm had be more concerned with time and/or space complexity on 1990s computers than I do.
If anyone's curious, the game's about 85% playable at this point (shop dialog is the last remaining hurdle for the player), and roughly 50% fully translated (the 229 Digimon bio's being a LARGE portion of the remaining text). There's a few additional problems related to increasing the Digimon's name max length from 10 to 16 (like running off screen and being partially overwritten in memory), but I haven't made enough progress on those to beg for help yet, haha.
Plus, editing the shops and title screen will keep me busy for a while.
Thanks again for all your help guys!