First let me thank you all for your time and say, in this first post, that I have no experience with breech seating rifles and am coming here from the Cast Boolits forum.
Please forgive anything that I haven't picked up from researching this forum and know that I appreciate your advice on this somewhat strange question.
To get to the point I own an H&R Ultra Slug Hunter break-action single shot 12 gauge rifled slug gun (built on a 10 gauge frame it's essentially a .73 caliber break action rifle). It's no Scheutzen, but it does operate on similar principles which is why I'm here.
I am no fan of plastic sabots and, as such, like to cast my own full-bore .731 slugs. Unfortunately, the Ultra Slug Hunter has a very long forcing cone which destroys full bore slug accuracy when the slug is roll crimped in a 3" shell.
Hence, my interest in breech seating. 
I plan on having an aluminum mould cut with a tapering diameter and breech seating pure lead slugs with a dowel fitted into an empty shell.
I then thought that I might roll crimp a shotgun shell filled with powder to seat behind the slug (should I roll crimp to the longest possible length or trim it to the shortest?).
I am open to either black powder or smokeless, although I imagine that bp would require a shell full of powder (way too much) and wouldn't respond well to plastic cases.
That said, I'm also open to using 12 gauge brass.
So, aside from an overall assessment of this idea, how would I safely load the shell with wads, cards, seals, etc.?
I imagine that having all that extra space in the chamber could potentially lower pressures for smokeless loads (the Ultra Slug Hunter is much, much stronger than a standard shotgun - I've shot plenty of 800 grain slugs at 1300 to 1400 fps).
Of course, I believe that having too much space between the shell and the seated projectile can destroy accuracy (most shooters breech seat brass 1/8 to 1/16 behind the bullet, correct?) and that the use of buffer, gas seals, and wads can lower the pressure on the barrel.
Basically, I'm looking to do whatever I can to maximize accuracy and minimize pressure. Incidentally, I'm familiar with a 1043 grain slug load that uses slow burning powder to reach 1000 fps at barely 10k psi. Subsonic velocity is fine and the load doesn't need to shoot minute of angle.
Yes, I know 1043 grains sounds a bit silly but slug ballistic coefficients are atrocious at anything below 800 grains and I'd like to be able to shoot to 200 yards.
Thanks again!

Welcome aboard.  I have no idea about 12 gauge breech seating, but my 32 and 33 and 8.15mm are all shot with a daub of 4227 in the bottom of a good sized case without any fillers or wads. Be careful with wads, ect.  They can ring rifle barrels. 

12 ga should be an advantage in our offhand matches; the leaded edge will be twice as far from center as a 32 

Again, Welcome.

Breech seating a tip up isn't real easy. I did it with a 30/30 in a 219 Savage. They won't hold still for you. 

If your really serious about this, have a bore riding mold made, with only 2 or 3 bands, about 1/8" wide or you won't have enough power available to get the bullet installed.

Now for the really bad news......... I once shot a 16 lb double, chambered in 8 bore, that shot a  1070 gr bullet. Your rifle will probably weight 1/2 that.

Pulling the trigger was the worse recoil I ever endured. I've alway described it like a strong, 250+ lb man, hitting me in the shoulder, as hard as he could!!!!

Good luck on getting the mold made and be sure to set a camera up and start it rolling, before you pull the trigger the first time.

Frank

Thank you all so much! I do realize that to really do it right I should probably go with a more appropriate setup. That said, I will take all advice under consideration and proceed in the safest and more responsible way that I can. Thanks again and I keep everyone informed!

In the late 50's - early 60's, there used to be a outdoor show, that had lots of African animals. 

It's been probably 50 years but, I will always remember one of the episodes, where they werre taking about the rifles for Elephants. The 600 NE was already obsolete but, was still held in higher regard over the 500 NE. In one sceen, one of the guys wanted to shoot the guys 600 and so, he sat down, aim and pulled the trigger. He went end over end. Does anyone else remember that? It sure stuck in my memory!

I believe the standard bullet for the 600 NE was 900 gr  and remember, that shot it in Doubles, lots more weight that shot guns

I want pictures,

Frank