Oneworld paid upgrades?
 

Which Oneworld airlines allow payment for an upgrade? Flights from US to BKK.

I have a new client who will pay for full fare economy, but not PE of J, so I am wondering which OW airlines can you pay for an upgrade, not with miles, but cash.

I know its very common to be offered to pay for an upgrade on domestic AA flights, but what about Qatar, JAL, CX, others?

At checkin / the gate or days before? An "upgrade" or just the full fare difference at retail value?
A full fare economy could be a very high price. More than PE or business class in a lower fare bucket. J is just one business class fare bucket. Could be JCDI
Can you deal with client's TA and pay the difference ?
Looked at which OW airlines can get from undisclosed USA airport to BKK? [Just QR JL CX]

For JAL, they offer cash upgrades to PE on day of departure. It's based on availability and the upgrade cost is fixed, not dynamic like AA. If you want to upgrade to Biz it would mean a ticket change and paying the difference in fare.
https://www.jal.co.jp/jp/en/info/inter/apopyug/