Google is rolling out an enterprise area of Google Play to allow for exact this scenario, but of course that relies on the company be using Google Enterprise services http://thenextweb.com/google/2012/12/04/google-play-comes-to-google-apps-letting-enterprise-customers-distribute-apps-internally/
Appart from that, remember that Android devices can install apps from anywhere, no need to be unlocked, rooted, jailbroken or anything like this.
You can just make the app available on the intranet website and make an API for update cheking,