Can a UL 1953 listed power distribution block be deployed in a panelboard cabinet?
Although this is more frequently done in commercial and industrial gear, I have never found anything against it in the NEC nor have I ever had a AHJ say they would be against it (I'm talking about true electrical inspectors in major municipalities). So long as you meet the requirements you mentioned above. The only problem would be if you would be required to submit the calcs that show they don't violate the fill capacity and you may have to make sure the connections were not exposed to incidental contact per NFPA 70E. 

The only problem I would see would be with the how UL would label it. As you know you are allowed to assemble control and terminal cabinets so long as you use "UL listed for use" material. But I have found a mixed bag of opinions from AHJ as to whether or not they want to allow that type of equipment that is not "UL approved" in their jurisdiction and the only way that I know of to get it UL approved is to submit it to UL and get them to approved it. It's obvious this is too expensive for the normal residential installation. The only other way is to find a manufacturer that has already submitted their equipment to UL for this type of modification.  

Personally I am for it. It looks cleaner and is looks more workmanlike. I have actually been a part of a group that we assemble "Main Termination Boxes". for commercial installations. Instead of a big junction box with a birds nest of wires and wire nuts and we never got any blowback from AHJ's. But in your situation I would talk to my inspectors in the area just to see how they felt before I tried it.

In conclusion your design would meet NEC and UL listed requirements but it would not meet UL approved requirements.