Depends. Different countries have different laws governing such. For the most part, if the hospital sees any legal liability at all, they'll do the standard CYA. Signing waivers / releases often doesn't do a whole lot, some of your rights you cannot sign away. Regarding your question, with releasing medical information, such waivers shouldn't be a problem, although the transplant scenario may be a special case.

Regardless of the legalese, transplant doctors typically get to know you quite well, and more information slips out (implicitly and explicitly) than may be allowed by law (HIPAA be damned). Nullum ius sine actione, as they say. If noone complains, noone sues. Bit like driving without seatbelts.