I want to be able to send postscript from my Mac 128k to the serial port so it can be sent to an Postscript enabled HP Laser printer that doesn't have LocalTalk.
There might be a 3rd party product that lets you do it. I've seen some that let you print from a Macintosh to a PC printer using a special serial to parallel adapter.
If your LaserJet has a serial interface, you might be able to get the LaserWriter software to cooperate. It's usually available as part of the System Software disk sets (after 2.x)