Install outside http root, possible?

Hi to all.

I have server space with a top domain and several sub-/addon-domains.

My idea is for all my sub-/addon-domains to install fckeditor just once in a folder underneath the top domain, i.e. "www.mytopdomain.com/fckeditor/".
In the subdomains I link the js file not relative but absolute with a external link i.e. "...src="'http://www.mytopdomain.com/fckeditor/fckeditor.js"></script>.
The advantage would be to install fckeditor just once for several sub-/addon-domains.
It start quite promising but ends with a "access denied" somewhere while initializing the FCKeditorAPI.
Is this a hopeless try from me or is there a solution?
Thx & best regards
manu