Is there a UNIX program that will convert between BinHex and MacBinary?
Yes, there are at least two that will handle BinHex 4.0, MacBinary, and other conversions. macutil and mcvert are distributed as UNIX shar archvies and can be found in the unix directory at InfoMac sites. Source code is included (of course) so you will need a C compiler to build the programs. Be warned all of these programs are old and are no longer supported by their authors.