Velcro won't hold with the recoil. Your only chances are:
- Make a mount to replace the rear sight (like some of the currently availiable glock mounts).
- Drill some threads on the slide and BBU to attach it there (I don't think it'll hold and I don't know if the BBU has enough material in it for that but...)
So far I've only seen 1 airsoft gun with the setup you want, and it was a japanese gun (so, less powerful gas).
Also, just in case you succeded (somehow), you'll be adding quite a bit of weight to the slide, so your gas efficiency will take a hit.
In short, no you don'