To do any expanding of the funcationalilty of the 128k like you describe you are going to have to do real programming ( Pascal, C or assembler ).
You couldn't be more wrong.
The way I read it you wanted the 128k to do SLIP or PPP. Both can be done, but you have to basically duplicate the functionality of MacTCP in a small memory space. You don't have Communications Toolbox or MacTCP so somebody, somewhere is going to have to crank up MPW.
Also, as Gorgonops deduced this is a OS X-side Unix solution not a 128K-side machine language, though it every well could be done that way ... why?
So install MacTerminal and you are done. Finished, end of project. Refering to a 128k is really just a huge red herring.