I can see it working if the game moves at a contant speed (probaly the map moving speed by default).
Battles would be done by only the person in a certain area can join or not (as they would have to move at the game speed time).
And castles and town seiging ould be done by the area when they start the battle not just click besiege the castle.
There would be no pausing, and if somebody recruits from a village somebody else can do so straight after.
And if somebody goes afk in a castle or maybe leaves the game, the AI could just play for them (they would wait in a castle I imagine).
Presuming you were playing LAN it may be easier, if it was online there might have to be some form of anti-lag in battles..
For companions I guess you would have one of each for every player, though it may be better to add in a few more and make the max each player can have to be 5 or 6?
Eh all just ideas - trying to keep the original game but replace some of the lords with actual people, OR have it only 2-4 players and have you all start as normal?