I could be wrong, but I feel like you'd be allowed to modify the special characters bitmaps in your project files (I already have, oops), but not in the engine file itself. Which should be fine, because there's really no reason to edit the engine's special characters bitmap, since the one in the project file is what's going to end up used in your game, anyway.
It still really baffles me that the special characters set was excluded from the Resource Manager menu. It absolutely seems like something somebody might want to edit, so that's definitely something I'd consider an oversight.
On this topic, it's also really easy to edit certain normally-fixed bits of system text, such as what displays for the Name, Class, Nickname, and State headers on the status screen. I'd definitely edit those for each project rather than for the engine, too, though. Each one can be up to eighteen characters long, including spaces and punctuation, as usual
F.Knight: We runs out the food! Hero: Anything could eat in this place? F.Archer: It's a bunch of Mushroom over there. I wonder it's safe to eat. F.Knight: No worries,we have to fill our inside no matter what it is.
(Hero & F.Knight Eat The Mushroom,After 10 Seconds They Collapse With Foam)