I don't know what terminal Jetstar uses at SIN, but there are transfer desks in the main SIN terminal (T1, T2, and T3 are connected airside and you can even walk) where DL uses T1. The transfer desks generally handle all full service airlines, although there can be several different dedicated agents, depending on the handling agent a carrier uses at SIN. I know I used the transfer desk once in connection with a DL flight and it worked, but I don't remember the details. I remember that they were able to locate and retag a checked bag efficiently and I was able to stay airside.

SIN is more complicated because if you originate there on a USA airline, some security questions are required before you check in, although if you arrive on a connecting flight, you can do the security questions at the gate.

Since you won't be connecting (due to the separate tickets), my guess is that you won't be able to use the transit desk but YMMV.

A local once told me that if you attempt OLCI (you won't be able to complete it and there's no warning, nor does it tell you why) and print the "not a boarding pass" thing, you not only go to a different and faster line at the airport (although SkyPriority might be faster anyway) but the check in and bag drop deadline is a little bit later. I've never seen this documented in writing, so I'm not sure I believe it, and I haven't tried arriving later at SIN. YMMV.

I don't see why you would need to wait to use the transfer desk if you can get to it, but my concern would be that if your flight arrives at the "new" terminal (T4?) which tends to be used by newer and LCCs/ULCCs, you would need to "enter" Singapore and then you'd be stuck landside until DL opens their check in counter. I'm not sure that any transfer desks in the new terminal (if there are, since LCCs/ULCCs tend to fly point to point and not sell tickets with connections) would be able to help with a DL flight at the other terminal.

OK, I had asked the OP to check the terminal for the inbound flight, but despite two requests, there was no response.

What I remember is each agent at the transit desk being used by a bunch of airlines; there wasn't a specific agent there for DL. The signage over each counter position indicated the airlines that were covered.

I think I once arrived into SIN on DL after midnight and I was with someone from FT who was able to get a boarding pass at the transfer desk for a connection the next morning before going to the transit hotel for the night. We were going to the lounge briefly either before or after the transit desk, so I tagged along. I don't remember the carrier the person was using; it might have been DL back to NRT as part of a MR.

Doesn't DL still do the security questions at the gate for connecting passengers? If someone has a boarding pass already, they wouldn't know to visit the transfer desk at SIN. I've answered their security questions at least once at the gate, but I don't remember when this occurred. It might have been SQ to DL on separate tickets coming from DPS.

Thank you. That coincides with what I did.