I've drawn many chain of custody blood alcohols. I absolutely refuse to draw against a persons will. In fact, I always specifically ask if they are complying of their own free will and understand the implications of their consent.(They are waiving their 4th amendment rights.)
Makes for a much stronger case if you advise people of their rights and make sure they clearly understand their consent.
Of course, most are too drunk to really give "informed" consent. They haven't a clue to the fact they they are handing over incriminating evidence.
Several times in the past (~20 years ago) I saw LEO's "bunny up" and come back into the OR, where they stood and recited something (not Miranda I don't think - but something else) then asked the anesthesiologist to draw for BAC. I always wondered how that stood up when the legalities came around.