How about this:
1. check that tmp.mount is active: if so we can assume that either tmp.mount
from fstab is used or customer has its own version in /etc/systemd/...
In these cases we can safely ship the default tmp.mount in
/usr/lib/systemd/system since the active version will take precedence.
2. if tmp.mount is not active: we still ship tmp.mount but we mask it
WDYT ?