Very interesting discussion. I began reading this a few days ago and started in the "can't do pilot assessment" camp but having done some reading of the relevant documents and re-read the thread my opinion has changed.
there is nothing in anything quoted above that specifically FORBIDS pilot assessment
Totally agree with this statement.
The section (paragraph 3) that deals with take-off RVR says in Note 4 that the reported RVR can be replaced by pilot assessment. The next paragraph lists EXCEPTIONS to paragraph 3 and those exceptions do not include a statement prohibiting pilot assessment simply a list of additional requirements for take-off below 150m. As stated above you would need to see 8 lights (type dependent) with 15m spacing to achieve 125m RVR.
We know that thick fog can be variable over quite small distances which is why the 90m segment requirement exists. You must have >125m AND a 90m segment before take-off, if you are given say 135,135,150 but you cannot see 6 lights you can't depart. Similarly, in my opinion, if you were given 100,125,125 but could see 10 lights out of the flightdeck you could depart quite legally. The rules simply state that the required RVR be achieved for all relevant points.
Of course individual airline managers may have interpreted this differently or perhaps don't wish their crews to assess the RVR in those circumstances and put a statement in forbidding the practice. Once that manual is approved it becomes the law for that operator but as far as I can see there is nothing in the raw regulations to stop you doing it.