OK, good news... I was able to solve the issue.
I believe I must have burnt up a trace from the SW B+ pin on the 18-pin connector that goes from the radio deck to the control head, so the control head was not getting SW B+. This can be verified by checking both sides of a blue 10ohm resistor close to the mic jack, on the back (component) side of the control head board. It should show somewhere around 13v, or SW B+. In my case, it was showing only 2-3v, for some reason... So, I jumpered from pin 17 on the control head connector to the bottom side of that resistor, as such:
I also jumpered from the bottom side of the one capacitor shown to the transistor which is a part of the OPT SEL/SW B+ sensing circuit. Depending on the current drawn through the SW B+ pin on the mic connector, the radio decides whether SB9600 serial lines get sent to the mic jack, or if standard microphone/audio signals get sent to the mic jack. If current is zilch, it assumes that it is a microphone. If current is > 2 mA (I believe that's the figure...) it decides that the attached device is a "SB9600 SMART" device, and sends the SB9600 to the mic connector. This is why a programming cable for the MCS has a 1k resistor from SW B+ (pin 1) to GND (causes 13mA of current to flow from SW B+, switching MCS into SB9600 Accessory mode). I found that I had no continuity between pin 1 of the mic jack and the 470ohm pullup resistor (possibly R0729) which supplies the SW B+ to the mic jack.
Like I said, the board layout is actually from a GM900 service manual (all I could find) but is *very* similar to the MCS2000's layout. Hope this helps somebody down the road! The radio is now working like a charm once again.
Are there any chip fuses to check, or does anyone know the fix for this off-hand?