Adobe has mentioned a use case of Knowledge Management in Multisite Manager documentation. CQ5 is a restful based service product, I don't see any problem in integrating with external entitlement solutions.
Here is some excerpts from Adobe documentation
ENT Corporation, a large distributed enterprise, has a large number of subsidiaries in various countries. All of them share a similar look-and-feel. These sites share various components and most are centrally hosted.
Corporate Communications at ENT use MSM features to propagate press release content to the various press release pages of each of the countries' websites. The press releases section of the source website is configured to be mandatory for all countries' websites.
Additionally, the Knowledge Management department at ENT has gathered a large collection of FAQs that apply to most countries. To allow the surfer to access the FAQ that is appropriate to his or her country, this content is configured to be optional on the MSM source site. As a result, the owner of a particular country's web site can subscribe to the content that is appropriate for that site.
http://dev.day.com/docs/en/cq/current/administering/multi_site_manager.html