Can Browning's Invector Plus chokes be installed in a Belgian made Browning A-5 12 GA magnum with full choke barrel?

No. Not recommended. The barrel walls are not fit for screw in chokes. Belgian A-5 barrels that had that procedure done in the 60's and 70's were mostly unsuccesful. The barrels ended up being cut down or discarded. Aftermarket screw in chokes worked well with the Over/Under Browning guns, but not the Automatics. It's recommended to purchase a Japanese made barrel (with invector chokes) and have it fitted to your Belgian gun.