I use the "lens" end of my GSO coma corrector screwed into the scope side of my 4 space / 2" filter slider. This means my scope can remove coma and allow eyepieces to reach focus in one go, as the coma corrector acts as an extension tube of sorts. Is there any reason the lens part of a barlow wouldn't screw into the telescope side of your filter wheel in a similar fashion? I have no idea if this would affect your ability to reach focus or not, or if it would fit, though. You would think the barrel and threads would be the same between a 2" barlow and a 2" coma corrector.