I guess we might be describing the same thing, and I am yet to have time to download and play with Monodraw (IT policies), but if there is indeed a way that surfaces could be replaced at a pixel level, so that for example the 'A' character becomes a Pacman, then we'd be aligned.

The only issue is, are these surfaces 8x8 or similar, and would it be possible to load in a 6x8 bitmap, for those unusual 8-bit computers of the era which used them .. I refer to my favourite system of the period, the Oric Atmos, which graphics techniques are described here: https://osdk.org/index.php?page=articles&ref=ART9

(EDIT: details on the charset feature, which would be 'nice to have' in Monodraw, here: https://osdk.org/index.php?page=articles&ref=ART9#title11)

IF I can edit the bitmap and render as 6x8 characters, Monodraw would be immediately useful for level design. In any case, when I have access to a non-work computer, I hope to spend some time digging in and informing myself, so apologies if none of this is relevant ..