A lot of work, and a complete recoding of the entire replay code, which records events available to BOTH players (fire events basically).
I could write up a new packet type for a spotting event and maybe lost-LOS in a few hours, and add it to the replay, BUT then both of you would get to see every spotting/lost LOS event for the both of you. and there would potentiaally be loads more spotting/lost LOS events to add to the replay than firing, so your data file size would max very quickly, a would replay times. Adding UI changes for it would be more problemantical since both sides in a PBEM game may have a different POV (see the problems already inbuilt to the engine if someone turns fast artilery OFF inside a PBEM ).
This would be a bit like the SP3 give away "*" that appeared on your unit's info to "telepathically" tell you an enemy unit could actually see you (or just had LOS to you, I forget), and which blew away such concepts as "ambushes" in that game engine. But on steroids!
A complete from the ground up rewrite of the replay code is one of the things that is simply not going to happen. Only worthwhile if I wrote a completely new game engine from scratch.
Cheers
Andy