I think you could probably make an educated guess at it just by looking at AA's destinations. Leaving aside the majority of (inactive) AAdvantage memberships, where are others likely to be?

I'd say the largest portion of foreign AAdv members are from Central/Latin America, Canada, and the Caribbean: places AA doesn't just serve, but serves frequently and from more than one U.S. hub.

After that, I'd put the next strongest overseas represenation in European countries with oneworld cities: London (ergo England) and Madrid (ergo Spain). There are probably also healthy colonies of membership in Brussels and Zürich, even though the LX codeshare has dried up. Helsinki I don't think as much, because AA doesn't fly there, and Finnair goes everywhere you could really need to go from HEL. (As does BA from London, but there's a huge volume difference)

There are probably also some accounts in Asia, but why would a oneworlder living in MEL choose AAdvantage over the Qantas programme?

That's how I'd proceed with my speculation, but... I think it's a fairly pointless question to be honest, and, much like the eternal smack talking about how many people live in EXPville, nobody except for people inside the supersecret Dallas headquarters are ever going to know for sure.