Any problem could result in a split barrel and possible injury to the shooter. Do you want this liability on your hands if you tell him it's safe to shoot a sabot through a full choke, somthing happens and he comes after you with his lawyer?
I don't quite think we're quite to the point where lawyers urged on by ambulance chasing clients are bringing cases based on discussion on Internet forums.
The nominal diameter for 12 gauge is about .729", but this varies widely in this day and age of backboring, older guns, etc. A really, really tight full choke will be around .690".
As one example Brenneke advertises their KO sabot at being .630".
A Google search on "slug split my barrel" or "slug ruined my barrel" might also be instructive. Be prepared to spend quite a bit of time before finding a page where somebody writes of having actually done this or having actually witnessed it. And while companies like Browning have notices on their websites saying that shooting steel through their older full choked barrels will cause ringing - but is not unsafe - warnings about shooting modern slugs through their old full choked barrels are most conspicuous by their absence. In the liability driven corporate world of the US, the fact firearms companies aren't posting notices that their ammunition products and/or firearms are sometimes incompatible says something to me.
Sabot rounds intended for rifled shotgun barrels would be the worst of choices, but the Internet is not exactly adrift with stories of split barrels. In a world of "faster is better", I suspect there are more than a few guys shooting sabots through their full chokes in search of the very best. Where diameters clash, my guess is the soft plastic sabot loses to the steel choke, but that's another discussion.
As Dutch already noted, generations of shotgunners have been shooting their full choked 12s at geese in the morning and then lying in wait for a deer that evening with rifled slugs up the tube. Many out there still do. They do it because it worked for their father and their father's father - not infrequently with the same shotgun.
As in all things involving firearms, common sense has a place in this. I wouldn't be particularly worried about shooting slugs through one of my brother's pogo sticks with a full choke if I felt a need to do that. On the other hand, I wouldn't consider putting slugs through my grandfather's old Davis SxS, purchased about 1918, with full/full chokes. While it looks to still be in mechanical very good condition with only surface wear, and was inspected by a knowledgeable gunsmith about ten years ago, that's another era of steels and 90 years of going afield.
The best choice with a full choke is probably going to be one of the rifled slug Forster designs, from an accuracy standpoint if nothing else. As I said previously, if you're really concerned, then have somebody mike the I.D. of the choke and then do some emailing and find the diameter of the slugs you want to use.
To go off on one last tangent, if I had a fixed full choke 12 gauge and didn't want to sell the shotgun, unless I was a turkey hunter or doubles/handicap trap shooter, I would have a gunsmith ream the choke out for me anyways, to modified or light modified, depending on what I actually used the shotgun for most. Or spring for thinwall choke installation if I wanted the widest possible flexibility.
In my opinion, aside from turkey hunting and doubles/handicap trap, the full choke was a lead shot longer range waterfowling proposition. With steel shot now mandatory for waterfowl, in most cases (including slugs) a full choke is an inferior choice for anything except ground sluicing turkeys and shooting doubles/handicap trap.