@Martyn ******
"Most"(?) Are you sure ? You mention Qatar. I flew Qatar to Thailand this past November. The agent at check-in noted my return flight was 90 days away.
I told her I was getting a 30-day extension in Thailand. She took the issue to her supervisor, came back, and continued the check-in.
On ARRIVAL in Thailand, the immigration agent never mentioned it.
This is ONE anecdotal experience, but I wouldn't state "most" airlines would allow it.