Just picked up the Sky90 from my friend tonight after work, of course, one cannot test a Herschel Wedge at night, but it's good enough to have some estimation first.
I placed a 1.25" mirror diagonal into the visual back, and inserted an eyepiece which requires more in-travel than other eyepieces, to mimic the effect of a 1.25" Herschel Wedge. However, I know such combination already requires less in-travel than the Herschel Wedge, maybe due to the fact that the prism (beam-splitter) inside is big compared with a regular diagonal. Anyway, if this combination does not work, it's not going to work with a Herschel Wedge.
This combination works marginally, the focus racked near all the way inside, just less than 5mm margin. Bad.
Later, I found the original Takahashi 2"->1.25" visual back has a very long optical path and I've one which is more than 1cm shorter, so I still have hope.
Herschel Wedge is somehow troublesome here, since you cannot put a barlow ahead of it to reduce the in-travel requirement, since the barlows will be burnt before the Herschel Wedge.
A side testing of tonight is to test whether the NGF-S can be used with the Sky90. If it's just to attach, yes, it can without problem with the 2" visual back. However, it lacks the required back focus so that none of the eyepiece will come to focus with a diagonal. For webcam imaging without a diagonal, it would be fine.