This sounds awesome in fact! I'd love to see a report and images of this afterwards
And I believe years back White dwarf did something with a big ol' empire campaign when all the goodies suddenly appeared (like the new Greatswords) and they did smaller matches where the remnants of that surviving army (in say an ambush) appeared in the latter game so you could even have much smaller games of say 500 against 1000 where the ambushee has to flee from the ambusher to make it to the big battle!
Also, if there happens to be equal numbers on each side, and whomever appears on board is random...How about each army selects one opponent to have them as their arch enemy? Say like it gives characters certain bonuses (Likely onl the General) if they meet in combat, which could mean uper cool fluff duels!
Have fun! ^^