Any reason not to do to this?   I would encapsulate the studs then add the rockwool, but am worried that the added width would make it to difficult to pack the bats into the space remaining.

Okay, had to reread this. Missed that you're not ACTUALLY encapsulating the Rockwool, as that MIGHT lead to condensation issues.
This seems like a needlessly complicated method to securing the batts.
On properly spaced rafters, your batts should fight snugly enough that they don't immediately fall out.

And if you're really worried about thermal bridging, add a layer of sheet insulation to the exterior of the roof.
Boom! Thermal bridging managed!