> I can't find the section right now but it says something like "no obstructing the purpose of your transmission". So theoretically you could add a header to every packet that says "purpose: testing hardware" and you'd be fine.

I think Section 97 is what you're referring to. If so, it's not obstruction of purpose the FCC forbids; it's obstruction of meaning.

Section 97: https://www.ecfr.gov/current/title-47/chapter-I/subchapter-D...