I'm not going to debate whether or not risk to occupants is increased or not with a cage in an 80, other than to say that I don't believe a blanket statement of "it's a bad idea" is correct. Without question, a poorly designed cage can cause significant risk, but I don't believe for a second that it's impossible to design a cage for an 80 that will provide better safety for the occupants.
My contribution other than my opinion above is to show some photos of the cage inside our Canguro Racing 200 Series Cruiser. This is a SCORE spec and approved cage for racing in SCORE and Best in the Desert Races. The driver/co-driver are in 5-point harnesses and obviously wear helmets and there are no rear passengers. The rear doors are fully functional however and the B-piller hoop is "at" the B piller and not further back into the door opening.
So take it with a grain of salt as it's not an apples to apples comparison but merely evidence of a cage that meets stringent safety requirements inside of a cruiser. I'm sure this thread will turn into a "spirited debate"

but just because there hasn't been a successful solution in the past, don't commit a new idea to failure before it even has a chance.
These aren't the best to show all the details of the cage but they are the best I have.
This is the best one to show location of the B Pillar and roof support
B Pillar harness bar and triangulation
A Pillar bars behind the dash rather than contouring around the dash. I doubt a bolt in cage will follow this approach.
Seat cradle and B pillar forward triangulation
I didn't bother with the C or D Pillar areas since they are of less concern.