Systemd incompatible with mounted /usr

Your questions makes a assumption which I am not clear is true. Fedora 14 includes systemd. It is not the default but it is easy enough to try (yum install systemd, boot with init=/bin/systemd). Fedora 15 branch includes systemd as default and I don't see any reason it wouldn't be released that way. If you want to try it out, Fedora 15 alpha release is coming out shortly

http://alt.fedoraproject.org/pub/alt/stage/15-Alpha.RC2/

You can grab a nightly build from

http://alt.fedoraproject.org/pub/alt/nightly-composes/