I'm imagining just installing GPSapp, plugging a GPS receiver into my car's DC power socket, plugging in the Empeg's serial cable, and having everything Just Plain Work (tm). Are we there yet?

Yes, that's essentially what I did.

Important to remember, though, that the serial cable needs to be three wires only: Gnd, Rx, and Tx. Because the in-car serial port has some screwy stuff going on with certain pins. And also there will be some trial-and-error involved in the correct "swap" of Rx and Tx.

And, of course, installing preinit and gpsapp was tricky because of the difficulty I had creating the /programs0 partition. But maybe by the time you get the GPS receiver, Preinint will have been updated to automatically create /programs0 for you. Just remember, don't have the WinZip smart CR/LF conversion on when you extract the preinit TAR file.