Nah... de-essing is not that mysterious.
It's basically a fast attack/fast release limiter, but responsive to HF content.
The Orban circuit appears to be concealing some of it's "secret sauce" to prevent easy copying (apparently a good idea).
The FET shunt is the full range gain element, with some distortion reducing (fraction of signal fed into gate) circuitry added.
Their frequency shaping (HPF or BPF) is inside their black box... It looks like they may be half wave rectifying before the shaping, I'd be inclined to filter before I rectify, but different strokes for different folks.
The nature of De-essing, only being active to squash transient sibilance events means you can probably tolerate a lower performance gain element (like FET) that is not in circuit most of the time. That said I would still invest in the distortion reduction, since de-essers are operating on the full range signal so you might hear distortion on LF content present during de-essing events. I have used FETs like that before in gates or peak limiters since the audio path is very clean when they are not active, and when they are active the FET nonlinearity is not dominating the sound quality.
There are several published descriptions of distortion reduction with simple FET element (old National Applications for one), and DBX has published papers about the basics of de-essing (in AES preprints).
Have fun, this looks like a good basic design project to get started on... Perhaps not worth literally copying the incomplete Orban schematic, but an OK general plan.
JR