I think some schools have guaranteed admissions programs for undergrads, but they might be for sophomores. It wouldn't really make sense to have one for juniors and seniors. I think Wake Forest and Vanderbilt might have programs like this (I'm not sure though...check out their websites)
I'm pretty sure most BS/MD and BA/MD programs cater exclusively to HS students. If one exists that lets you join after, you probably need to be enrolled at an affiliated school.
BA/MD programs exist so that schools like ___ (insert poor school here) can soak up really good students early instead of losing them to Hopkins, Harvard, Michigan State University, etc. once they apply to medical school... (the last one there was just to see if you were awake) If your already at some other university, it defeats the point...
Hmmm that's why Northwestern has such a program right?? And I'd like to also add that the 7 year candidates are not always the best ones. Some don't even get to stay in the program at USF because they couldn't break even a 28 much less the required 30 even after taking the test multiple times. Nor do they have the profiles of a HMS accepted student or even half the profile of them in all cases. Definitely true at USF.