Almost certainly not legal. Plus, you're looking at only 3 rounds, max (plus maybe one up the spout).
The Thunder 5, on the other hand, is legal and lets you mix 410 shot shells and 45 Long Colt.
And if that looks too much like a kludge, how about the BFR 45 Colt/410, from the folks that brought you the Desert Eagle?
Of course, if you feel you need 12 gauge, I prefer the Neostead.
It is set up with two magazines that are selectable, meaning you can switch easily between "less than lethal" ammunication, like the stuff from Blammo Ammo (I love Dragon's Breath) and the real thing. The dual feed system also gives you 6 + 6 in the tubes. More than sufficient for any home invasion situation. In fact, probably sufficient for any Al Qaeda encounter. If you need more than that, you need more people backing you up.