Access rights applicable to page sets

It would be nice to manage access rights for specific page set instead of individual pages. I suggest to build access mechanism on rules. Rule would consist of two parts - matcher and access flags. For each page displayed/edited/... rules will be consulted to modify access policy derived from wiki and space. Individual page settings would override access settings determined by rules.
Rule matcher may be of folowing types:

• page parent (as described in XWIKI-1091)
• page name prefix
• actual page class
• page properties
• arbitrary groovy code snippet

I can try to implement such access control on my own, but since I am new in XWiki, I would appreciate any hint you can provide.
Usage scenario:

We use XWiki to communicate with clients while working on projects. Each project has its own wiki space. In the space, we want to mark some pages as private - i.e. visible only to our company, not the client. Private pages may occur in any position in space page hierarchy. Therefore using extra space for private pages in not possible.