Now that I think of it, I really regret I didn’t make systemd ebuild install it to /usr from the very beginning. But the harm has been done already, and I’d like to move it ASAP and that’s why I’d like to sum up problems with that and possible ways of proceeding with it.
The idea is simple as it is: move systemd install to /usr prefix completely. Right now, there are no technical benefits from keeping it in rootfs. It already depends on libdbus, which is installed in /usr, and I expect more dependencies over time. There’s no reason to move all those packages into rootfs.
Most importantly, the above information allows me to assume that such move won’t hurt our split-/usr users — because they already had to have /usr mounted for systemd to run.