I don't know why not Mike, but it's not my call. It's certainly not mentioned in the work information. No reason I can think of why they wouldn't want to do it.
As an aside, MB does state that the work should be done by a professional welder. We do the inspection of the subframe in the shop. If there are no cracks, we order the gussetts, then set up an appointment at our body shop to have the work done. When the parts arrive, we take the car to the body shop, along with the parts and a paper copy of the work instructions and have one of their guys do it, someone with lots more experience with a welder. This can then be billed to the work order as a "sublet repair" under warranty.