I'm not proposing anything since we talk "in principle", not about anything specific. What I say is that the current technology is not final and anything could be possible and it is an engineering problem unless it's restrained by the fundamental laws of physics. Could be hard or impossible with the current state of the art but that's why we do research and development, aren't we?

"single finger prick drop of blood at levels that consistently correlate with venous levels" is definitely not one of the fundamental laws of nature. They may not correlate but maybe it's possible to acquire the information about the body through some other channels. It's not like there's a single way of surveying the body, right? A specific test can be impossible but another method can be develop for obtaining the same information.