It’s surprising how little attention is given to traffic passing in the opposite direction. I have tried using a closed and rapidly opening fist towards the top of my crash helmet, ( to represent flashing), and have been warned accordingly using the same method, but many would not understand the message being conveyed.
On my trip to France last month a group of ten pals,were returning to England on the same day, on a similar route to me. In fact we passed each other on a long straight stretch of road between Vernon and Pacy sur Eure, and despite me flashing and waving, none of them saw me. So the chance of quickly conveying the presence of cops, other than a headlight flash, which of course is illegal in this country and certainly is in Spain, will be difficult I think.