Oh, I am heading in those directions.... I haven't formalized anything yet, but I was thinking that direction.....
What I am picturing is something along the following lines:
Say your character has a Will of 15, this would give him a Mental Strength (MS) of 45 (I am still debating between making it 3x Will (for full psi user; 2x fo semi, 1x for non/wild), or of creating a Static number based up type of psion user.
Our psion user is a BioPath, so he focuses mostly on psions that enhance his own body. Combat is coming, so he decides to activate Armor. The TN for this is 18. He rolls a 34. This is 16 points above the TN, so he gets 1 to his AR for beating TN, and the +1 for every 5 points over TN (this also gives him a Boon point), so for a total of +4 to AR, he has to pay 2 MS for base, and +1 MS for every 5 points (or additional +1 to AR), for a total of 5 MS. Our Psion used DOES have the option of activating to a lesser degree (i.e. pulling his punch, as it were), but he cannot activate higher than he rolled. The Boon Point - he can use this to either increase the psi effect by 1 or to decrease the MS cost by 1 - he decides to increase the psi effect.
This means that so long as he has Armor up and running, his MS is reduced to 40.
Our Psion also decided to learn Energy Bolt (outside of his BioPath abilities). This lets him throw what appears to be bolts of electricity. Since this is outside his BioPath, he had to learn a separate skills for using this. So, he rolls and makes his attack, For this, he has to decide how many MS he is putting into before activating it, as this determines the Damage Rating (added to his Will stat bonus to determine Base Damage). Our psion user decide to put 3 MS into it (with his Will stat bonus of +2, that gives a Base Damage of 5. He rolls, and hits his target, getting 7 point over foe's DEF, so he does 1 extra point of damage...
Okay, so our psion user is now down to 37 MS. Each round, he will recover 1 MS, until he is back to 40. He cannot regain those last 5 while using Armor.
However, should he become stunned or unconscious, those psi abilities that he was maintaining will automatically drop. If he is Dazed, then he has to make a save vs Will (TN 15 + # of rounds Dazed) or drop the maintained abilities. That is unless he has enhanced Armor with another ability that makes the maitenance of the psi subconscious until he deliberately shuts it down (this basically increases the MS cost of a given Psi ability, but cannot be used on all abilities).