Probably in general people shouldn't be using the built-in Playdate OS keyboard for any long form typing, but... I've managed to find one use case!
As a result, I've realized that it could be quite handy to have "key repeat" function built in to the keyboard. In particular for me, holding B to continue deleting (for large amounts of text) would be incredibly handy. I don't see a way to access the input handlers while the keyboard is open, so there's not much I can do to solve it myself.