Hi Roland,
thanks for your reply!
$ vgchange -a y $ mkdir <mntpt> $ mount /dev/<vg/<lv> /<mntpt>
As long as the FS to be mounted is not needed for anything systemd related, this should do it.
This is what I ended up doing, though in the slightly more refined way outlined in my first message: the rc.boot script doesn't presuppose any partivular LV or mountpoint, but simply looks for relevant fstat stubs. Moreover, it also adds export entries and starts the nfs server, if appropriate. The per-host configuration is then handled by the fstab stubs, and these can be distributed individually with the softgroups mechanism.
A.