Don't worry, it's not a brick.

USB-serial adapters work just fine for kernel downloads etc, but I don't know how well they work with the Windows-based upgrade software.

To unbrick your player, re-install the Hijack kernel using the Linux based download program (or perhaps using LogoEdit). Then use the Linux based upgrader utility to install the .upgrade file over ethernet, avoiding the USB/serial port problem.

Cheers