Mounting a directory using NFSv3 over IPv6 through Security Gateway, the traffic is not matched to the relevant rule containing the NFS services (instead it is matched to a general drop rule).
Mounting a directory using NFSv3 over IPv4 through Security Gateway works correctly (matched to the relevant rule).
Mounting a directory using NFSv4 (which does not require the use of a portmapper) over IPv6 through Security Gateway also works correctly (matched to the relevant rule).
Cause
Support for NFSv3 over IPv6 (Sun RPC services - rquotad, mountd, nfsprog, nlockmgr, pcnfsd) is missing from relevant definition file ($FWDIR/lib/tcpip.def) on Security Management Server / Multi-Domain Security Management Server.