Using this on earth is hard because any clouds prevents this from get into extreme temperate ranges. So, you have low utilization in the 10-30% range.
This also gets vastly harder the further you get from the Sun and the asteroid belt is not all that close. A solar concentrator + solar panel + more traditional methods may work better especially out in the ort cloud.
Possibly, but you can make a kilometer diameter mylar mirror which folds up into a pretty small pouch. A small puff of gas to inflate the 'struts' and poof you've got more sun to work with. The asteroid belt is nominally 2.5AU from the Sun and using the inverse square law would suggest 1/6th the amount of solar insolation. So the Odeillo oven has 2600 sq meters of mirror and generates temperatures of 3000 degrees. Our 1km dish would have 785,400 sq meters so about 302 times the surface area, with 1/6th the insolation we should get about 50x the energy out of it if my math is correct.
Aiming a 1km dish is not going to be easy not to mention station keeping. Also, while it's clearly producing a lot of heat and may even vaporize iron that's not the goal. Further without Gravity many approaches for refining materials don't work as heating a chunk may just eject it into space.
Alternatively if you look at it as a large and relatively cheap tool it may be useful as a relatively small part of the process.
This also gets vastly harder the further you get from the Sun and the asteroid belt is not all that close. A solar concentrator + solar panel + more traditional methods may work better especially out in the ort cloud.