Thanks for those tests, Peter.
So, to summarise:
- Take your favourite sideloaded font family (with good coverage of your required character sets), which normally looks a bit lightweight on the Kobo, especially with the frontlight on.
- Rename the 4 font files, inside and outside (using a font editor), as Delima or Rockwell or Felbridge or Times New Roman (maybe others???).
- Sideload to \fonts and reboot Kobo.
- Use your favourite font just like a fully adjustable Kobo font.
- Keep fingers crossed, they don't remove the option in the next fw update

Sounds like a nice new trick for those who need it and who have access to a font editor. I'll double-check later with a couple of my favourites.
Update: Yes, it seems to work OK.