Turns out lxc supports specifiying relative paths within its mount
entries, preventing duplicating the rootfs path.
# rtc
lxc.cgroup.devices.allow = c 254:0 rwm
-# mounts point
-lxc.mount.entry=proc /var/lib/lxc/template/rootfs/proc proc nodev,noexec,nosuid 0 0
-lxc.mount.entry=sysfs /var/lib/lxc/template/rootfs/sys sysfs defaults 0 0
+# mounts (note that the second item in each list is the mount point, relative
+# to the rootfs)
+lxc.mount.entry=proc proc proc nodev,noexec,nosuid 0 0
+lxc.mount.entry=sysfs sys sysfs defaults 0 0