In the scenario you mention, libvirt should probably do a sanity check for
this before letting you start the guest. libvirt already supports the idea
of 'shared' disk images where two or more guests can be optionally configured
to have write access - basically assumes the admin requesting sharing knows
what they're doing.
I think this is the right level myself. Advisory locks work okay but
not all filesystems support them. It's particularly nasty when you have
a clustered filesystem in the host. I think it would do more harm than
good to have a feature like that was supposed to provide a safe-guard
but then frequently didn't work.