Hi, I'm brand new to the helix but I have a pro recording background.
You have a multi (3) band compressor in Dynamics. You should be able to set the low and mid band up so they do nothing and then tweak the hi band to clamp on esses.
It's not quite the functionality of a deesser, and it's a shelf rather than a bell curve - but it will get you into that zone and may be usable for your proposes.
I agree though, a dedicated deesser has FAR more cool uses than just taking spit out of a vocal and should be implemented, esp. as there's a mic input here.