It'd be extremely useful to permit mock to run under systemd-nspawn, instead of chroot. Notably, this would let mock be used to run simple package test scripts that could start and stop daemons, which isn't currently possible under mock with systemd. This is just down as "hey, this'd be nice".
Systemd is not present on RHEL6 (which we still support), but yes - it would be nice.
Implemeted as f05982c. in py/mockbuild/util.py is variable: USE_NSPAWN = True when set to False, mock will use old code for classic chroot. Spec file substitute that for EL6 builds.
One additonal commit b792207. And I find that some src.rpm have assert on building under root user. I will try to add user into that container.
One more commit e6017d3 with uid handling.
Fixed in mock-1.2.0, which just landed in rawhide.