I made a COPR for git builds of niri: https://copr.fedorainfracloud.org/coprs/yalter/niri-git
Apparently this is a thing you can do; relatively easily, even. It's fully automatic, triggers on a webhook from GitHub.
Thanks Michael (my4ng) from our Matrix channel for showing me the relevant docs and doing the initial work!
niri has reached 3000 stars yesterday!