Testing revealed a funny bug:
Wired in the following way to use a single bulkhead plug:
30 Clutch IN
86 Clutch IN (as a +12v source for the relay coil)
87a Clutch OUT
85 WOT switch (to energize the coil and cut off the compressor)
Wired in this manner, when the clutch is disengaged, the WOT switch will find a ground path through the relay coil, over through the relay switched terminals, and through the compressor coil. This will be low enough resistance to permanently put the Jetronic control box into enrichment mode.
I spliced a blocking diode inline from the WOT switch and the Frequency Valve went back to normal.