[plt-scheme] Read and write ports from pre-opened file descriptors?

Hello, everyone. I'm relatively new to scheme and I've been trying to  
writing a binding between Erlang and Scheme for a clustering  
application I've built in Erlang. Part of this entails reading and  
writing to file descriptors 3 and 4 in a binary protocol for  
communication with Erlang via its Ports abstraction.

It seems like this is possible to do at the C level, but I'm not sure  
how to do it with MzScheme's ffi library. It would be best if there  
were no external binary objects. Could someone suggest a way that this  
might be accomplished?

Thanks,

- dlf