Update... came out pretty darn good, I think. My wife didn't really
I ended up using 1/2 chimney chimney of lump charcoal in 1 weber basket all the off to one side and two pieces of hickory on top the coals. It was probably a bit too much as heat was hard to keep under 250F. I had to almost close the bottom vent and keep the top vent mostly open to vent the heat. Briquettes probably would have been better for lower/slower portion of the cook.
Took right about 50min to hit 125F and I opened up the vents and let the coals flare up. Charred up the onions and the burgers a bit two at a time and then moved them back to the cold side and put the lid back down to let the cheese melt. My wife likes hers well done, so I just cooked hers longer over the fire at the end.
Got some really bold smokey flavors. It was definitely different than a smash burger or a plain grilled burger. Almost baconish. In hindsight I think different cheese than american is needed ... it wasn't bad but it even two slices barely peaked through. A bolder cheese like an aged or sharp cheddar, a swiss/guyere, or a bleu cheese would have been better to counter the bold smoke. Yeah, a reverse seared smoked burger with bleu cheese crumbles in the mix and topped with some balsamic-tossed greens would be great.