Has anyone hiked or made the trek on this trail from Disappointment to Muzzle or anywhere in that area? I'm wondering if I could portage through here to Muzzle or if it's just not going to be established enough to do that without bush wacking which I have no interest in doing.

I would assume there isn’t anything to follow. Probably more like a bushwhack along game trails if you’re lucky. I’ve only done it in the winter though.

Satellite imagery supports the map's indication that the area is boggy. It may be passable in dry conditions but the eastern end, approaching Muzzle, would definitely require bushwacking and the path is much less clear.

Edit: Following the Kekekabic trail and bushwacking north from the T-intersection near the east end of the OP screen cap would be more plausible route. Still about 1000 feet of bushwacking though.