Whoa. That's cool. A while back I wondered if we could just synthesize the proteins externally, but this way we only have to figure out how to synthesize, package and deliver mRNA and the process could be used to generate an arbitrary set of proteins. Presumably the proteins themselves are going to vary as much as the tissues in the human body, so the synthesis might be the same but the packaging and delivery parts would seem to vary widely based on characteristics of the protein.
To the best of my knowlede, the Novavax vaccine actually does this. Instead of delivering an mRNA nanoparticle, it delivers a spike protein nanoparticle. In fact, they just started phase 3 trials: https://www.sciencemag.org/news/2020/12/novavax-launches-piv...