Well, the nice thing is that all the maps are already label Dog 1, Bravo 2, etc…
So, the first part of the notation for a hex could be something like D1 (Dog 1), B2 (Bravo 2)…
We could start with a standard then on numbering the hexes 001, 002, etc. Maybe say that the corner of the map that has it’s name Dog 1, etc… the first valid hex on the top row is always 001 (meaning no half hexes).
Since we have a tendency to flip the maps around the hex numbers won’t always be facing the same way as the other on the map, but still saying your M2 Flamethrower moved to D1-024 (Dog 1 Hex 024) should be easy enough to figure out.
Then for facing we could always assume that we write the hex number at the top of the hex so that could always be position 1 for facing. Then we could just number going clockwise at that point.
6 / 024 \2
5 \ /3
(Sorry for the crude hex… I’m at work…)
So, the M2 Flamethrower moves into this hex and is facing the bottom of this hex (4). D1-024-4 (Map Dog 1, Hex 024, Facing 4).
Anyway, that would be my guess how to do it. I’ve never played a play-by-mail game so maybe there is an actual standard for this, but this seems like it would be easy enough to me.