I'm not familiar with 777 pellets, but I am familiar with ASM Dragoons and Walkers.
You can certainly stuff 50gr of powder into a Dragoon but you might want to be a little more conservative. All it takes is for that wedge to be not quite seated all the way after firing a few rounds and you're going to "alter" your fine engraved Dragoon. Also, the dovetail for the loading lever latch WILL eventually loosen up after repeated heavy loads. If you're lucky you will notice when the loading lever falls like a Walker, if not you'll be on your hands and knees looking for that sucker in the gravel.
Personally, I wouldn't use 777 in a Dragoon for heavy loads, nor any other gun for that matter but that's just me, you do what you prefer. I think you will find that 30-35gr of 3f (even Pyrodex:barf
works really well in the Dragoon and you won't notice the difference in smoke much if at all. If you want availability to shoot without carting around all your shooting supplies, try rolling your own cartridges, a few dozen in a tupperware and you're good to go anytime.
Also, why don't you post a pic of the Horse Pistol, I've never seen an engraved ASM before, it's nice to know they are out there.