We have a range of visibility in the map right? Why not only allow roster applications for those who fall into that range; in other words, allow to the clan who has a member who travels with the fighting army.
Edit: Or let me put it this way. We see nords are attacking templar fiefs. Let's assume Nord_Ismelllikesalmon attacks Templar_Iamaninfidel on the map. By default, Nord and Templar clan members would be allowed to apply for roster. And assume that in the visibility range of Nord guy, there's a member of DRZ and Grey. Then, these clans would also be allowed to apply. But clans like a thousand miles away would not.
I believe this might increase the depth of diplomacy in the game as you'd need to ask for your allies to send one of their member with you to the frontline for them to join the battle on your side. Only after this, clans would show their true strength.
Extension: In case of attacking neutral villages, the roster application might work like old-school way.