I searched for sinit on the gentoo-user archives and couldn't find
anything, there's only a few Reddit threads which don't help much. Would it
be possible to follow the steps here[1] to use sinit + daemontools-encore
on Gentoo, or are there hard dependencies on OpenRC that make this
impossible?

This is completely for fun. I'm aware of discussions of this init vs. that
init and I don't hate systemd/OpenRC or anything like that. Again, this is
just for fun.

I'm willing to do a lot of work myself to make this work, but if Portage or
some other core Gentoo subsystem doesn't play well with sinit or
daemontools-encore it'd be just a waste of time.

[1] https://troubleshooters.com/linux/diy/suckless_init_on_plop.htm