According to Wolfram Research Technical Support, the Macintosh CAN be used as a
front end for Mathematica running on the NeXT computer.
To do this, you need Mathematica for the Macintosh and the NeXT. Wolfram has a
document called "Macintosh TCP/IP Installation Guide" -- free to registered
owners of Mathematica -- that describes how to use MacTCP and Mathematica to
connect to the NeXT computer and how to use the Mathematica kernel on the NeXT
computer. To get this document, contact Wolfram Research Technical Support.
For more information, search under: "Wolfram Research"