It would help only if the following architecture is valid:
[Synth PC][USB A] ----> [USB B][USB hub #1][USB A] --> long cable --> [USB B][USB hub #2][USB A] --> [USB B][Keyboard with USB MIDI out]
This is because I need to preserve the single (non-host) USB port on the keyboard, and the (host) ports on the PC too.
According to the original USB design specs I vaguely remember, the above is supposed to work, but I have never seen it tested, especially with MIDI; anyone know?
Of course, I could just use male-to-female extenders instead of hubs. I will probably do this in the short term, until I have better :-)