Thanks for the replies:
The original question was a flag of convenience, so I was assuming foreign-based/operating N-reg. Not American Airlines!
"Authorities" may well be able to find out the owners behind a trustee-N-reg, but what about airfield operators? Doubt that. Try pursuing a Delaware corporation for an unpaid fuel bill, landing fee or perhaps an insurance claim?
Ironically, if you wanted to hide you could do it much closer at home: try a Scottish limited partnership with an Cayman-style overseas company as one of the partners. (Alleged to be very popular wth Russian money-launderers!) .
So, yes, I agree that cost of maintenance is - in most cases - the main driver of foreign N-reg. But it also has a certain reputation!
ps. For disclosure: We actually own our airfield through a Scottish LP, but have dropped the (non-foreign!) limited company as a general partner!