"ECON is SPECIFICALLY prohibited by international law."

Not really.

On certain Pacific routes we fly econ all the time and so does every one else. When I fly the SFO- FRA or LHR we fly ECON except on the class II nav sections and then we use a fixed mach, as prescribed by the route and the WOM.

Don't be such an alarmist. In fact many at UAL over the years have flown ECON as long as it matched the planned and cleared MACH. The problem is you must stay in that % range of the cleared mach on certain routes, Atlantic being a critical area, or else you are in violation of the regulations for that route.

It is a whole lot easier to request a clearance at say .85 and just set it there rather than having to watch the ECON Mach to make sure it doesn't violate the cleared speed requirements.

I'm basically lazy and hate writing reports, so on my flights we fix the mach where it's required. If I was a F/O I would insist upon it and write an IROP and an FSAP if a Captain refused. For sure take it to Professional Standards.