In this case, it's a matter of tracing an unobstructed (no enemy units, no uncrossable terrain) path from where the reinforcements are to where they are to be used. Given that, it boils down to the same thing.
There are ways to partially simulate fog of war in a tabletop game (face-down counters, including a scattering of decoy markers), but it only goes so far. For instance, you could have a one-sided fog of war (one side's counters are face up), but not a situation where one side is unaware that the other side has omniscient-eye intel (which appears to be the case in this battle).Erfworld has a fog of war, so a tabletop game is really ruled out from the outset unless you have a referee. I think this is why I haven't played a table top game in over 20 years, wth the possible exception of SPI's monster War in the Pacific, which a friend had coded into his Macintosh so that we could keep better records of what was going on. That game was so big that it tended to generate its own fow.