Since I asked but didn't see a response, I figured it might be helpful to someone else to share my experience a few days ago: arrived on an sq operated flight and departed on a ua operated flight, on separate tix at T3 DEL.

Since UA-assigned staff at DEL had previously (months prior upon inquiry) advised me to let my arriving airline know about onward flight connection on separate ticket and that they would "hand off manifest and issue me a boarding pass", I decided this time with a 3.5 hr connection to try this approach.

Sq staff at check-in at SIN seemed unfamiliar with such a process and said they would "add a note on my ticket" so staff in DEL would know I needed assistance at the transfer desk.

All airlines besides AI staff the international transfer desk on an ad hoc basis.

Multiple gwr staff at DEL told me that all airlines are able to send agents to the international transfer area on an ad-hoc basis as arriving passengers needing international transfer arrive. However, beware that if you have any sort of Indian visa gwr staff may encourage you to exit passport control/customs to the checkin counters, which may not be desirable.

In my particular case, though nobody from sq told me to expect a ground staff member (the ubiquitous Sats employed ones in orange jackets) to greet me at the gate, one was there and caught up to me at the international transfer area. She made a call to united. Eventually two of their staff showed up, one asked security questions and the other issued the boarding pass. Including clearing security, it took 40 mins from arriving in the international transfer area to clearing security into T3 for my ua flight. Considering how quiet it seemed at the international transfer area, this time to clear may be a best case scenario.

Hope this helps someone else.