I found this on Spooky Tooth and I'll measure the bike up over the weekend. I noticed in the thread you linked to as well.
I know it's going to be close on their 11" measurement. Between the bottom bracket and top tube it's about 13" but the bottom of the triple triangle is lower. I don't know how much. However, most of the motors I see mounted have the cylinders tilted a little forward which might get me a little more room. Am I wrong?
I figure I'll probably have to come up with a drawing complete with all the dimensions and run it by the vendor before ordering.