I know I can get a transportation letter from the US Consulate in HK
http://hongkong.usconsulate.gov/lpr_faq.html#a2
to get me back to the United States. But of course, this is only good if I am there..... USCIS wasn't particularly helpful when I called them today - they told me that the airline has the right to check my travel documents, but that's all. As far as I know, the GC isn't even a travel document - my passport is.