I have a Moog EP-3 running into my Soleman and it works fine for sending midi cc through the neuro hub. It has a switch on it that changes between standard TRS output to some other configuration, so it would probably work with a 1/4th to 1/8th TRS cable straight into the c4. The SA expression pedal is probably a safe bet, albeit twice the cost.
I'd wager even a $20 Nektar expression pedal with a 1/8th TRS adapter would work, or if your midi controller has a 1/4th TRS input it would probably do the job as long as the midi controller allows you to assign the appropriate channel (ch. 100 by default) Though you might opt for a mini expression pedal for more pedalboard real estate.
The pedals, the neuro hub, and the Soleman all have an expression pedal calibration tool in the desktop editor as well. So finding something that will do the job shouldn't be too hard.
*edit* went back a page or two and saw you mentioned the DMC Micro, and reading the manual, I'm thoroughly confused on how to properly setup either a tap tempo or an expression pedal using the multi-jack on the DMC Micro to a specific CC# that you can assign on the c4, but plugging a 1/8th TRS into the c4 should work for expression input if the DMC Micro doesn't allow for this.
Looking at the specifics for the Eventide H9 in the DMC Micro manual might be a clue, that expression output is assigned to CC# 48, and tap tempo is assigned to CC#44, so you might be able to set the c4 to CC# 48 and use the multijack on the DMC micro for expression control, or plug the exp pedal into the c4, and plug a single switch for tap tempo into the DMC multijack, set the c4 to CC#44 for tap tempo control, since the H9 allows for changing the CC#, which the c4 is capable of as well. But I could be mistaken.
IF you end up with multiple SA pedals and a neuro hub, the neuro hub has expression input as well.