Is there a specific sysex string I can use to definitively bypass DSP A, and another sysex string I can use to bypass DSP B?  Or am I missing something

Doesn't really work like that. You choose the CC that you want to use, and tell the unit about it, as described. Some setup is required. Otherwise, you can be sure that the CC we wanted to use would be needed for something else.

For sysex, look at the document I attached – but it is a lot of work.