US citizen services responded yesterday.

They can forward any state absentee ballot or federal write-in absentee ballot to the US via diplomatic pouch.

The ballots must be in a postage-paid envelope for mailing within the US.

They warn it can take 3-4 weeks for full delivery, so check your state election board website for the absentee ballot deadline (also, many states allow you to vote via email or fax these days).

To use the consulate service, mail your ballots to:

Attn: ACS/Vote
美国驻成都总领事馆
四川成都市领事馆路4号
中国 610041
(28)8558-3992