I don't think "opt-out" is a good (or sound) idea -- but even if it was -- it'd be pretty hard to enforce: take an auto-accident. You shouldn't be delaying emergency help, just on the off-chance than one of the many victims might be an "opt-out". It would complicate triage for no really good reason.

Then there are things like contagious diseases; you would have to spend money enforcing some kind of quarantine in order to cater to the "opt-outs".

For all other cases, like most types of cancer -- you can opt out -- only you'll have to commit suicide (I don't advocate this either).