Thanks for your comment.
Yes, hostbased ports would probably do the job - although I'm not completely sure, if there are multiple connections from one inside client to various shares on the same outside server, they should be mapped to different originating ports, I believe, and I wouldn't know how to handle that.
Anyway, all that requires much more insight into IP than I have. After some research, it turned out that most of the shares I need are also exported as samba shares, I use that instead of NFS now, and it's working reasonably well.
However, I believe that this is quite a common problem (since VMware has the simple options for it). Would be great to see a better solution than move to a Windows implementation.
Thanks, Frank