Default Serial bootloader commands
The serial debug command set is slightly disappointing:

In fastboot mode with serial, it doesn't seem to accept any commands.

I can't see any difference between "RUU" mode and "fastboot" modes. You can enter RUU mode with the oem "rebootRUU" command. It says "RUU" on the menu instead of "FASTBOOT".

In "HBOOT", or bootloader mode, it only accepts the "oem" commands I've listed in this thread.

All the more advanced commands are still compiled into the ROM, its just that nothing ever calls them.

Interestingly, I found out that if you hold down VOLUMEUP on powerup, you enter the ARM9/radio bootloader. It accepts commands over serial only, but has responded with "invalid command" with the few things I tried.