Hi Ben,
See the problem. Well here is a discussion from 2009 where sudarsan shows a way to mount a read-only .vmdk on linux by hiding it behind a read/write child disk. Though this approach involves compiling the VDDK sample application etc.
Maybe this approach could help you. As far as I understand things, I could imagine that you might be able to create a local child disk (read/write) which points to the read-only nfs-export base disk. Then simply try to mount the child disk read/write.
The concept of child-disks and disk chaining is layed out here.
Hope this helps, altough not really quick fix ;-)