Why not just store the scripts in the safari.extension.settings or the local storage, and write the extension to load arbitrary ones? Add in an interface similar to UserCSS (
http://code.grid.in.th/ ), and I don't see what else you'd need. Though you could add install buttons to userscripts, and script auto-updating, and a whole mess of whatnots to it if you wanted.
(not sure what'd be fastest / the best option, I'll leave that up to you to find out)
