I didn't say there was a sidechain in the DA-7 Joe. But you can create a sidechain of sorts.

Mult a vocal track in the DA-7. One channel has to be used as the main signal that's ultimately passed onto the L/R bus. The other for the EQ source to compress the main vocal.

Take both vocal channels out of the L/R bus. Assign the EQ source vocal to bus 1. The main vocal to bus 2.
Then enable the compressor across the bus pair and be sure the bus pair and compressor is set for "stereo" operation.

Take bus 1 out of the L/R bus {so the EQ or control track stops there}... make sure bus 2 "is" assigned to the L/R bus. Then set the compressor threshold on the bus pair, just above where it starts to compress.

Go back to the main input channel that's assigned to bus 1.
Enable the EQ and with a semi-narrow Q, boost 4, 6, 8k or whatever. Now... when the threshold of the compressor at the bus pair is exceeded with the EQ boost, it will de-ess the vocal assigned to bus 2.
Again... considerable gymnastics but works in a pinch.