May not be a BAD idea but that's a fairly major shift for archers to take. What they have now is, if the RevDCM page Bug option is on, is Archer Bombard. Sounds like you may be aware of that already. My point though is that collateral damage is done with an actual attack by a unit that dishes out collateral. If I'm not mistaken, this includes siege weapons, as in no unit is doing 'collateral' with just a bombard.
Archers and siege weapons aren't really something I envision coming in to attack at the front of an army anyhow. So what part of the next Combat Mod phase will include will be Support Promotions. These were developed elsewhere on the site and I think they will be very appropriate to include soon. To explain it in short, it means that when you have multiple unit types in a stack, each is adding a bit of its own element to the unit making an attack. Therefore, if I have Siege weapons in my stack and I attack with my swordsman, that swordsman will have a support promotion that gives it some collateral damage, for example.
Extending on the idea would be sets of skill promotions that grant enhanced support promotions so you can develop some units to be 'better' supporters in the actual fight than others.
But back to your suggestion, you seem to be saying we should add a little collateral damage on a successful bombard attack as a possible effect and I don't see that being a bad idea. I'll have to check to see if this is already happening on siege weapons now but I don't think it is. If so, however, I'll see what is making it work that way and then take a look at what we can do to work that in to archers as well.
One thing we might wish to do in general is give archers at least the ability to earn skill promos that give them some collateral ability too.