It might just be easier to make a new travel failure notice for the smaller army. "The roads are blocked by an incoming army" or something to that effect, not unlike when you run into a dead end and it adds four hours to your travel time. I think I've had something like that happen to one of my characters while he was marching with his own army, in fact.
I suppose this doesn't solve the problem completely since the smaller army or unit would still be in Travel mode and not fighting, but it could at least delay their movement as they have to find a way around the van, and grant a strategic advantage to the incoming forces since they can now swing around and give chase.
