I know the first gen 246 boards (a and b) use a regular spec ps2 board with a Namco daughter board, so they would be equal to a retail ps2 in spec.
As far as the 246c and the 256, I do not know if the usb ports are even active in the system since they use a JVS I/O on them.
The 246c should be the same spec as the earlier versions but just a single combined motherboard which integrates a ps2 main board and Namco daughter board into one.
The challenge to this would be to run the system in such a mode that would allow an external source to have access to it, I do not know if that's possible, but of course that's all speculation.