The book already has a great mechanism for that: enchanted items and potions.
And the reason "free" shifts don't work is that you don't get something for nothing. The idea is game breaking. Lets say i rely on some items for offense and defense, leaving mental stress track free, so at the end of every scene, i dump all into this "item". So what about every scene not described. Assuming each scene is 15 minutes, thats 96 shifts per day. Minus what you spend. Game Breaking. That is over 100 shifts per scene that you *could* throw around every day. After a week of not doing anything, you could get nearly 1000. As a GM, no effing way. not unless every baddy could to. And with them sacrificing people, 10,000 per week easy. Nuke. 50 shifts to a small city for an hour... Can your wizard survive that???