I've never heard of a 7 rd mag for the mini, only 5 or 10, 20, etc.
Like Yossarian said if a 10 rd mag. could be blocked off for example a wooden dowel just under the base plate? I would check with a gun smith for ideas. There are some states over here where you have to have a plug in a shot gun to limit shots to 3 rnds. This isn't perminant, but it must be in the gun when hunting. If caught without it your toast.
BTW I'm moving this thread to the magazine section.