if i could get my modem to work on linux i would test it, But i cannot see why nfs cannot access fat partitions. If it cannot this is because fat partitions do not use inodes and every part of the linux file system is based on them.
The linux kernel has dynamic inode creation built into the fat layer to fix this problem. This enables the VFS to access it via it's inode_structure.
but one would thing nfsd interfaces with the file system via the VFS, meaning serving via fat should be possible.
But nfsd tends to not follow mounts, meaning if u serve a directory bellow the mount point u will only gain access to the blank directory not the mounting point.
i noticed this on my server as i had a 120Gb mounted on /home/download and mounting /home did not give me access to it, instead i had to mount it as well.
just in general if nfs interfaced with the VFS there is no way mounting a fat volume would be a problem and 2nd there is NO WAY in hell linus would allow nfsd to directly access anything else but the VFS.
So in concluding it is possible to mount a fat partition u just have to explicitly mount the fat mounted point.
hope that helped