Appbrain would probably be the best option as they offer an API: http://www.appbrain.com/info/api
The best way to do it would probably be to add a button somewhere that would open a pop-up where you could search either Appbrain or Playboard in order find and embed information about an application.
Or maybe fetch ID from the play store URL: https://play.google.com/store/apps/details?id=com.gameloft.android.ANMP.GloftFWHM
Use it to gather the relevant information via either:
I'm sure that there would be a way to do the same for Apples app-store. Probably better to separate them as two different add-ons though.