So, I asked about this at the airport upon arrival...

The UA flight i was on showed a bunch of BR, TG and VN flights on its Connecting Information. I suppose it is because we had passengers connecting to these flights. At TPE, none was offered OSS and the UA agent did not know what I was talking about.

I then asked an airport employee, he did not know what I was talking about.

Then I think later someone told me maybe this works for same-airline only and UA apparently sometimes has people doing SFO-TPE-GUM.