Have not done that, but, I did shoot a bunch of light loads made as follows. All shot "beer can" sized groups at about 50M and did not lead bore.

¤30-60 grains of 2F black powder or same volume of 777Fg in primed case
¤54 caliber soft fiber wad, lubed & seated on powder. Separate lubed wad from powder using a non-lubed wad if not going to shoot almost immediately (to stop/slow lube getting into powder).
¤54 calib pure lead ball forced into case and pushed down onto wad -- this will create a band of 0,515" or so around ball, which should be good fit to groove-to-groove of original 50/70s and 12,7X44Rs.

Also works well with 12,7X44R rollers and same caliber barrel of Husqvarna, etc. kombi guns.

Grisen

Additional wadding is not the only workable option, although that is a time honored method. With "straight-walled" cases, many shooters load lighter loads with the bullet/ball entirely inside the case. This is pretty safe with BP, Pyrodex and 777FFg, but, be very careful with nitro/smokeless powders, especially with fast-burning "pistol powders", because you can quickly end up with chamber pressures higher than the rifle is designed for. Unless you know for sure what you are doing, I would stay with BP, Pyrodex, 777FFg, etc.

I much prefer to load bullets entirely inside longer cases than to load them into shorter cases because the longer case fits the chamber for normal full length, as the gun was designed for. This especialy for rifles.

Grisen

Are you sure that a sabot will clear the throat and leade of a chamber without snagging up on something? Maybe no problem, but it might be worth thinking about first. Hope this gets to ya in time.

I was just suggesting that you might check it out- just in case. Don't think I was trying to say it wouldn't work. Just urging a critical assessment of fit for all the components. The sabots for muzzleloaders don't have to negotiate the beginning of rifling in the barrel, they are already placed in it. Just a thought.                   Have fun.

No, I guess you aren't the first. I hope you let us hear the results.

First shots out this weekend were at 50 yards.  The sabots came off with no problems and landed at about 25 yards away.

The group is nothing to praise about as it was my first 5 shots since I put all the parts together but I think once I get this horribly long trigger pull issue fixed the groups will get better.  The group of 5 landed at the top of my 2'x3' target at about a foot above my bullseye target so I'm thinking those stock sights are regulated for much longer distances    The group was horizontally spread at about 5" but only a 2" in height.  That to me, means that my trigger pull was erratic and I could use some sharpening up for the next time this Trapdoor comes to the range.

The plus is that the 200gr Semi-wadcutter 45ACP bullets cut very nice sharp holes in the target board.  No tipping or instability noted.  The sabots showed engraving from the rifling and I think I've found my base load to build off as the recoil of these "carbine" loads was lower than what I would expect from a big cartridge like this.

Cleanup was pretty easy as well.  The sabots meant no excess lube carbon to deal with and the residue of the pyrodex was easily flushed out of the barrel with some hot water and some scrubbing.   

I then re-oiled all the metal parts and the barrel and reassembled the rifle.  Is it overkill to disassemble the breechblock for a water rinse and clean it as well when shooting Pyrodex in a TD?  I don't want to score up the breechblock's face with any residual powder rust from shooting.