Yes, you're right. The real issue with retrofitting chain stay mount (low mount as Paragon calls them) dropouts is getting the stays out to the front of the dropout. If you go with seat stay mounting, this might be a little less depending on what it takes to clear the disc. The old-school vee brake mountain bikes tend to have decent clearance for the disc, so maybe the low mount dropouts will work. I feel like a road bike would pose more of an issue unless it's for 140mm discs. Which is to say that someone that doesn't do this all the time is going to have to do a lot of fiddling beforehand and I can understand why they wouldn't really want to do it or at least charge a reasonable rate for their time.