I like the idea of an on-call system, but I suggest doing it differently.
Instead of desperate dwarves calling for a relief workers, I'd like to see the power in the players hands. Being able to schedule this like the new system for guard duty, seems like a natural progression to me. If you want to put your dwarves on duty until the stress makes them start throwing things and starting fights, that's your prerogative.
While a dwarf is on call, he should get a very minor unhappy thought, that lasts until he is released from duty. "His free time is currently restricted by his work," or something like that would be good. When you work in an industry that forces you to be within five minutes of you employer at all times, lest your services be needed, it puts a constant mental strain on you that can and does make people irritable and even violent. The power of this thought should build up ever so slowly, as the work period goes on, and the overworked dwarf's nerves start to fray.
When he does get released, he should get a small happy thought, like "He is reveling in having his freedom returned to him." I've worked in a position like this, and believe me you don't know how great a weekend really is, until you've gone without for a few months.
Dwarves in on-call careers should take an extended break, after their on-call period ends, as most positions that require you to give up your entire life to be available have some form of off-season or sabbatical system, to rehabilitate the exhausted worker, and keep him sane. After that, they should go back to taking jobs, even if they aren't "on-call". An option to disable this extra break, on a per-profession basis, would be cool.
This gives players a reason to train more than one or two people for each profession, in order to concentrate all the XP in one place. It's never been realistic, and this is a step in the right direction, especially with jobs that only require occasional work in DF, like an ER doctor.
It should probably tie into the burrow system, as well. It doesn't matter if we have three doctors on duty, if none of them are in the hospital you got rescued to.