Please forgive me if this is misplaced. It is specifically about OAL, but it always involves a flight on AS so that seemed like a common link making this forum relevant. The example used here is BA, but it can easily apply to CX, JL, LA, and others.

Background
As I live near a very small airport (PUW), I am used to having issues purchasing tickets to some destinations as airlines won't publish a fare originating here. My usual solution is to drive to GEG and fly from there. But I much prefer flying from the local airport with 3-5 daily flights to SEA.

When looking to book a PUW-SEA-LHR flight on BA, I noticed that BA does NOT publish a PUW-LON fare. Additionally, only select First (A/F) and Business (C) fares seem to be eligible for End-on-end ticketing. This apparently means that I can't combine a PUW-SEA leg onto the same PNR despite having flights available that would make good connections.

This leads to the cheapest PUW-LHR itinerary on AS/BA being >$9000 (in F, but unfortunately my company won't pay for F)

My question
I've heard around FT occasionally that "a good TA should be able to combine a domestic segment onto an international fare even if they don't sell it directly." Is this actually possible when the airline adds the "no end-on-end ticketing" restriction to nearly all of their fares? The agent's I've talked to can't seem to do it.

I've had similar issues trying to book other international routes where only full-fare tickets allow combinability.

Any suggestions? (besides driving to GEG)