But wait, even better: with a second "device" for GPSapp to open/use, Hijack could just "disconnect" the player (blocking all serial input/output) whenever it sees a userland app open() the second device for use.

The only thing NOT caught by this would be the boot-up spewing of stuff out the serial link, and possibly a few lines from the player before GPSapp (or other userland app) gets going and opens the second device.

But that's probably not a big deal. And all this would allow me to rip out the serial port menu entry (no longer needed) in favour of this new automatic method.