I too would really love this feature, it's one of a few that would allow me to work entirely in Affinity's apps. Currently I just go into Illustrator and throw down a shape, do a pattern fill, and expand it. Copy > paste it into Affinity, create a mask, and pop it into place. It comes in as vector and everything (doing this on a Mac). Some cleanup is necessary, and it's really not ideal but it works for simple uses.
Since there are several threads of this same topic going around, someone also linked to this:
https://www.lostminds.com/patternodes2/
I'm considering using this as an option and am giving the trial a shot, would be great to have something similar built into Affinity itself.