Thought I’d update this topic with a solution, of sorts - it’s not a way to get bitmap fonts working, but it does give you the ability to pixellate UMG widgets for a ‘retro’ style game. Basically if you use a retainer box, then the child widgets get pushed to a rendertarget and can be manipulated via a custom material. By pixellating the UVs of that material, you can make the effective UI resolution much lower.
I have an example of it in my PSX shader pack, available free on github here.