I first saw the magazine release doubling as a bolt catch on the ADAC lower receivers by AXTS Weapon Systems and found it highly useful. Expanding the same functionality, albiet with an external spring, is the Redi-Catch, a product from Redi-Mag. 

Using a S-shaped steel sprint, anytime the magazine catch is pressed, a spring pushes against the bolt catch. When the charging handle is pulled, the bolt will be locked to the rear, a highly useful combination for clearing a double-feed malfunction or unloading and showing clear.

Redi-mag has two videos of the Redi-Catch in action. One as a stand-alone device and another used in conjunction with the Redi-release, a BAD lever type system.

Those interested can pick one up on their website.