You cannot tie mountd(8) to specific ports or port ranges on this OS. There are workarounds and alternatives available; see the thread in the misc@ archives that begins with the first post in the link below. Along with solution discussion, a number of posts reiterate the broader message regarding the lack of security for NFS over insecure networks, such as the Internet. As with the original poster you desire to move your data in plaintext beyond a firewall, which assumes over an insecure network, and this is considered a
bad decision.
http://marc.info/?l=openbsd-misc&m=115092459119047&w=2