FWIW I've run various systems with partitions replaced with MFS (usually copying from HD at boot and syncing back to HD at shutdown). This makes sense for some directories but I came to the conclusion that doing this for the rest of /var is more trouble than it's worth, in particular it's easy to get out of sync between /var/db and files installed on persistent storage.
My recommendation if you want to reduce active disk writes there would be MFS /var/run (and /var/cache if you have things that use it), but FFS for the rest of /var, use syslog memory-buffers for "standard" logging (and if there are more important things, you can easily keep writing them to /var/log on a facility/level or per-source basis). syspatch should give correct messages in error cases but I don't think the behaviour otherwise really needs to change.
