Following a suggestion by MaxWilson, and making use of DrPraetorious' excellent work on spell modding, I've made a mod that effectively allows you to disband units.
What it actually is is a version of Flames from the Sky which costs no gems or research, and only affects your own units. So, you just need to herd your chaff into a province, make sure you have noone important there, and fire away.
Initially I had it only as a holy spell, before realising that only mage-priests can cast rituals, and not all nations have them. So now it is also an N1, F1 and D1 spell, so everyone should be able to cast it one way or another.
I've made two versions of the mod. The one attached to this mod should be seen as the canonical one. However, the flavour text is a little dark, so those role-playing a benevolent god may prefer the second version. It will be interesting to see which gets more downloads.
This is the text for this one:
Name: Final Sacrifice
Description: "Even the bravest and most loyal of soldiers may one day cease to be useful to their God. Perhaps they are injured, diseased or simply too far from the front lines to make a difference, and finally they become nothing more than a burden on their empire. It is to these servants of the new God that the priest offers the opportunity to make a final sacrifice. He calls down enormous divine fireballs from the sky, composed of a magical fire will only affect loyal servants of the new God. The fire will incinerate them utterly, such that even in death they make one last contribution to the war."