I have GDB running between two Linux boxes with a serial interface cable between them. I can set breakpoints, single step and do source level debugging in the kernel from one machine to the other.
Unfortunately I have not been able to make this generally available for two reasons. 1) I have been unable to do any Linux work at all for several months due to other pressing needs. 2) I am a beginner at the Linux kernel and toolsets and do not yet know how to use the "patch" facility.
I hope to be able to get this info out there before too long.