Thanks for all the suggestions. I think the way I'm going to develop this is as a stable of ghost soldiers who can turn up depending on circumstance. I've just googled 'G Company' (for Ghost Company) and found out it's a Texas military living history unit (soldiers pretending tobe other soldiers from history as it were), so that seems like a good enough justification to call the idea 'G Company'. And I'm working with all your suggestions! It gives a kind of callsign for the unit and means that when they get back to base and report that they made contact with G Company there can be that idea of file gradually amassing of the file of contacts with a non-existant unit.
So the next question would be... I have about 80 more ghost soldiers to name, anyone else want to help?

No, really I think I'll kinda run with about sixor seven of these - Captain Jackson is too good to miss out on, and the psychic echoes of Corporal Kowalski (and his little dog) even made it to Discworld so he's definitely in. I'll work up the biogs of GI Joe, Kilroy, John Doe, and 'Iron' Mike Irons, hopefully making one a sharpshooter, one a close-combat specialist, one who knows about explosives maybe... and if I can get Jeb Stuart and the Haunted Tank in there too, I will!