@Zukow While you do not start making hack-version of SDL, if you have an interest you can follow there: https://github.com/dreamer/dosbox-staging/issues/144, the man who tries to fix it inside of DOSBox invite you as well, so we all can test/discuss things there.
I mean, it surely better to fix DOSBox than making some hacks in SDLs.
EDIT: Oh, and he was able to fix things the way you suggest, just he change different PMAKEs in some other places. See: https://github.com/dreamer/dosbox-staging/blob/77bff33870aed1622df871820e91ba470bb9f676/src/gui/render_templates.h
And result in all 16bit modes and 32bit modes in a window and in fullscreen rendered correctly on our side now: