/var on tmpfs

[Don NetBSD <netbsd-embedded%gmx.com@localhost>]

I've a box with a DoM.  I'd like to mount / as ro and create a
tmpfs for /var (and /tmp).  I don't think anything else NEEDS to
be rw (the infrequent changes to /etc can be made by unlocking /
to make those changes).

I imagine I can just make a tarball of a skeletal /var and
unpack this over /var, once mounted?

Is there a preexisting mechanism for this sort of thing?
Or, do I roll my own?