I couldn't find the unit produced by Mossberg, but if you will call them I'm sure they can help.
I did find this:
http://www.adamsonindustries.com/progard_a7.htm
It's holding a red shotgun replica and uses a remote electronic release button. A handcuff key can release it as well or you can have it fitted with a regular key overide. It is aimed towards law enforcement, but it might be of use in this application.