I exchanged some emails with the astrocrumb guy, who quickly pointed out that as the Skywatcher spider lines up with the focuser a slide cannot be used.
Probably the easiest mod would be to move the plate that the focuser is installed on just enough to clear the spider vanes. We'd need to drill four new holes, tap a thread into them, and cut a new notch in the eyepiece plate to clear the spider vain. Only question is whether I could do it with the required degree of precision. It's not like I can stick that whole thing in a drill press vice.
Is this something a machinist would be equipped to do?
Edit: on further consideration, it shouldn't be a difficult thing to do, a jig could be built to facilitate precise hole drilling, and cutting out the notch would be 15 minutes with my Dremel.
I'll continue to exchange emails with Astrocrumb guy, and see if we can figure out a technical solution.
Any other 20" Skywatcher owners interested in a purchase if we figure it out, i.e. for group buy purposes?