On October 22, the U.S. Senate passed S.1390, the Defense Authorization Bill. The bill includes the Military and Overseas Voter Empowerment Act, which requires states to mail overseas absentee ballots in federal elections at least 45 days before the election. This will almost surely force states with September primaries for Congress to move them to late August. See this story.
In the original version at least, there is a hardship waiver for states whose primary date would preclude transmission 45 days ahead of time – but it may be impossible for States to actually gain the waiver.
It is a requirement under the bill that States transmit ballots, etc. electronically if requested by the voter, and that there be a single contact point in each State.
So ballots can go out quickly, what is needed now is for the federal government to provide rapid transmission of voted ballots the other way. As it is, States are only required to count timely postmarked ballots received 10 days after the election.
Let’s say that it takes 4 weeks each way. Then the voter will have to vote as soon as they receive the ballot.
OTOH, if the polling location were physically overseas, then the number of ballots cast could be sent back to the local election officials who could simply wait forever how long they needed if the votes could be decisive.