As it happens the Ottawa area G1MRA membership includes a very accomplished artist who I'm pretty sure would be willing to help (and who may in fact be reading this and chuckling).
My options do seem boil down to 1/ completely repainting the model using the paint I got from France or 2) coming up with a DIY colour match for the Aster paint. The local Hobby shop certainly has plenty of Humbrol paints to work with.
Lest anybody following this thread have an interest in another source of SNCF green, I've come across one. See:
This is a water based acrylic. I don't know how this might impact a post office's view of it, but it has to be much less hazardous than the enamels that Precision Paints can't ship or (surely much worse) the lacquer paint and dilutant that I got from France.