I guess you could do it a number of ways:
1) A simple-ish (but perhaps rather brut force way) is to allow the user to just type the name of a style and add it to a list, which is filtered in the style dropdown. But there might be potential problems with spelling errors in the style list, etc and of course you'll need more functionality in your admin area.
2) Allowing a developer to add a special "SnippetMaster" style sheet as a comment much like you can already add snippet comments in the html to define areas to edit. This special stylesheet could then be read and picked up by your tool, thus filtering the unwanted styles. Perhaps a bit more work for the developer?
Well, you're the genious - I'm sure you'll figure out a better solution!
Thanks again, I think I will use your tool and donate at some point!
Looking forward to this feature being added. I'm sure it's hard to balance having a simple tool and also satisfying all feature requests!