Paid position at a US lab Can one get paid while conducting research at a US university lab, without being a staff, student, visiting student, or a postdoctoral fellow? What are possible titles that are acceptable by grants? Assuming the grant sources are NSF and IARPA.

According to wikipedia, visiting scholar is a high level recognition. Is there an alternative for a paid researcher who does not hold a PhD and is not initially affiliated, but whose research is valuable to the lab?
Given that most universities have different titles for different functionally similar positions, I wouldn't sweat it too much. The only places where things are different in my experience with NSF are for faculty and post docs. I know NSF has a rule about the total amount of time that a person can be funded (without explicit exception from the Program Officer) on an award (2 months), but these rules are generally only applied to faculty since they are expected to get 9 months of support from their institution. NSF also requires a Postdoctoral Researcher Mentoring Plan for any proposal that will fund post docs. This is there to ensure that post docs have their careers promoted appropriately and that they learn appropriate collaboration methods.

That being said, you don't even have to work at a university to be paid under an NSF award. There are frequent subcontracting relationships created to send funding to other institutions including private companies and contractors.