Using a different WORK folder for ports

[Bigbrother]

Hi people,

I want to NFS export read-only the ports tree /usr/ports to different
fbsd machines. When the clients try to build a port, the compilation
fails because they cannot write to the ports tree. 

Is there any hack that it could redirect the ./work folder of every port
to be a local rw disk,
like /usr/tmp/work ? Thus, patches and compilation would be redirected
to a local hard disk. (This would increase also the compilation
performance)

Thankx in advance!!