This whole thing kind of reminds me of when I worked for the Census last summer. Latino folk would have to check off the box for Latino but then also check off the box for Caucasian. I got a few dirty looks from that when I said I had to mark them down as both.
I know right? People had to fill this out for a program I worked for and my Mexican descendant co-worker was like "WTF?" She didnt' want to have to check "Hispanic" and "Native American" also but that was the only other thing besides "white" (which she didn't consider herself to be) that made any damn sense. However, she did mention she had family that lived down in Texas (also Latino) that claimed to be white. And I will say when I was living in South America it was a MUCH bigger thing. White people all over the advertisements/in power, and my SO who is biracial (black/white) found that people down there who were darker than him considered themselves white. Spanish ancestry is to be thanked for that conundrum.
Anyway, to the OP, I would say check with adcoms. Don't say your name and even use a new email account if you want to, but I would send this all in an email to them so (1) you have something in writing, and (2) you don't have to explain the whole thing over the phone which would potentially lead to them giving you a gut re-action response. I would imagine that the whitest of lily applicants of Mexican descent (of which there are certainly many: check your celebrities folks) could claim URM...OP if you're in a position where you are like what I stated above (mixed background ancestry, I don't know why you wouldn't be able to claim this even if you're considered white in Libya because you have "lighter" skin). Just like Hispanics in the US, you could very well be labeled black or hispanic in this country but white in another. And if law schools are accepting people from Jamaican and Carribean decent as AA, then that label has been stretched already.