What is the best solution, if any, for carrying out LVM style filesystem snapshots on OpenBSD?
Looks like your OPENBSD solution might be rsnapshot
rsnapshot is a filesystem snapshot utility for making backups of local and remote systems.
Any integrated feature of LVM other than doing the snapshot/restore may not be available, unless some other tool provides it.
Interestingly enough, rsnapshot is the example used in the OpenBSD FAQ on it's packages and ports.
Give it a try and hopefully it meets your requirements.
It's built in and called CCD. Look at section 14.18.1 of this FAQ page. It mentions a man page as well, but I can't link to that since I'm new.
EDIT: Not sure whether it supports snapshotting, but it may be of use.