As a UK and a US dual-national living in the UK, I find that the system will allow me to OLCI to the US but not from the US - I keep the US passport in UA's system. One check in agent (I think at SFO) told me that there were a significant number of regulars who were in a similar position - because the system essentially was not designed for people who had a residency separate from their citizenship. And yet, of course, a very significant proportion of UA's international travelers are in that very position - I read somewhere that there are 250,000 US citizens living in London alone.

As to needing to see an agent, this is fine at SFO when traveling non-stop to an international destination as they do at least employ agents. When traveling from the domestic terminal however, it's a nightmare. My recent experience (traveling in F as a 1K) was that all the elite lines were closed and I had a 45 minute wait in the general check in line. Reminds me what it's like to have no status and why such people hate UA with such venom.