If you set /etc/platform to "nanobsd" then it should do the memory file system md0/md1 stuff, and the packages list will show you the packages that are suitable for the nanobsd platform. It might all work, but doing a full install and then changing it to "nanobsd" under-the-hood might have other effects, good and bad - e.g. Diagnostics:NanoBSD will probably show up on menus, giving options to duplicate boot slices… The option to periodically backup RRD and/or DHCP lease data might be handy though, if it works in such a config.
With SSDs these days, there is a use-case for having a hybrid install that can put a full install on a chunk of an SSD, then also select the nanobsd-style features that are desired - putting tmp and var in memory, periodically backing up useful stuff from tmp and var to the SSD, preserving various things during a clean shutdown...