Good post, good idea. Have some "outside force" disrupt excessive turtling (but not too much please, i like turtling )
As nathaniel richter already said: the topic starters idea probably does not jive with the extreme scalability the devs have in mind. It would be very hard to implement it properly that way and also feel "rigid" and "meta-game"
Perhaps a good idea to keep in-line with the background story: In the last cataclysm lots and lots of energy got stored up and then went boom, basically. So take the permisse that our channelers are not quite as good as "the old ones" were. Any excessive turtling would probably at one point involve hure stockpiling of mana supplies or putting into ever increasingly powerfull magic items.
Back to the premisse: our "new" channelers at one point start to lose control over that much power. You get "random" blowouts, the chance of which increases as the excess gets greater. So at one point you have to start letting it out and doing something usefull with it or suffer the consequences (and I do believe that there are legion effects one can think of to get to that point: to much magic attracts demons/dragons, flareouts destroy terrain, flareouts "burn out" caster units who are even less in control then your channeler, magic items might get overloaded and go boom or BOOOM, magic frenzy might hit and turn a numer of troops in a certain location against you, fighting for independence).
Basically all this allows for a certain amount of powerhording but doing it too much will cause you damage. You might even make it have effect on other players realms (but less so) as to encourage them to find ways to get you to expend some energy (not wait untill you blow up).
It might even be used as a tactic in an MP game - See other thread: debalancing death effects - Faced with (near) certain defeat a channeler might start hording power to produce a new cataclysm. If I cant win - we all lose kinda thing. Other channelers would be forced to attack him/her to prevent that from happening (after all: they still have a chance of winning) (and pushed to that - not only by end defeat but because once the hoard gets large enough its start to create random effects in their backyard too). This would force other channelers to attack the "hoarder" under unfavourable circumstances - his terrain/his domain on his defensive positions. If the hoarder gets lucky it might even be he inflict sufficeint damage so he becomes a viable force for a while longer.
This would prevent the need for something artificial like a "counter". The increasing occurance of "blowouts" should be indication enough and I'm sure channelers spy on eachother - it would be rather hard to keep a major blowout hidden.
Depending on which way you choose to develop you might be able to harness more or less power but always within limits
edit: i agree with above posters: AI last ditch efforts I have rarely (never?) seen done well. Either the AI is a risktaker or its not - Switching back and forth (recovering from last ditch if it happened to turn out well) is - I think- not at all easy to program. So many variables.