I know this is a little late for the spell casting/concentration issue you were having, but we play tested a mechanic in my game this weekend, and I figured I'd share. In the party we have a Wizard, a Warlock, a Fighter, and a Rogue -- so not much in the way of healing, which may be why this worked out OK.
I allowed our caster to concentrate on one additional spell at the cost of 1 HD per round. If the HD was spent on concentration, it wasn't available for healing during the short rest. If he went beyond the amount of HD available to him, he began burning levels of exhaustion each round. It provided a nice trade off, and an easy way to track, since I give everyone tokens for their HD that they give to me when they spend them. They get the tokens back at the end of a long rest. (anyone else notice how much people like Some kind of physical representation of stuff?)
It played out OK. First big(ish) combat, he buffed himself and the fighter with Protection from Evil/Good. Combat went fairly quickly and all was well. They didn't bother with a short rest. But then they got ambushed by just a couple of goblins. The Wizard got hit during the surprise round for a fair amount of damage, and even though it was an easy fight (except for the surprise) they needed to do a short rest, during which the Wizard suddenly realizes he has no HD left to heal himself.
The next fight and the fighter was all buff me again, and the wizard was like... ummm, sorry maybe I'll save that for when we Really, really need it. So, basically it hasn't seemed to break the game or anything, because now he seems really reluctant to do it. Now that may be because they don't have easy access to a lot of healing magic, so we'll see what happens if we have someone in the party who can heal.
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Btw - I liked someone's idea of a level of exhaustion after hitting 0 HP. I added it, cause it's pretty great. I'm also thinking about adding a mechanic of reducing Con bonus by 1 with every critical hit taken (which would reduce Max HP). It seemed more straight forward than the Unearthed Arcana Vitality rules - plus, less math - and it makes critical hits a little more scary
not sure to make them removed with the spending of a HD, a long rest, or something else. Thoughts?