@jonne I am a man of my word: consider Sprites::drawPlusMask implemented.
I will wait for someone to confirm that it works on the simulator and/or (if possible) a genuine Arduboy before merging the PR, just grab a copy from my local repo.
I have attempted to get a simple sketch running on my own, but I keep running into missing symbols, e.g. delay, PROGMEM, and for some odd reason the vtable for Print… general Arduino stuff.