Thank you both.

They were very definite on the Messenger chat about seat pre-selection not being possible for tickets issued via Singapore Airlines. "Since your tickets are issued under [618] and not [390], you will be provided with Business Class seats; however preselection is not provided". That tallies with the 618 number prefix on the SQ issued tickets. My conversation with them has been going on for several days due to message response times. As a result it has gone round in circles somewhat and I've had that same answer twice, probably from two different agents.

Let's see what the call centre agents say...

I've also been talking to them about a separate OA flight that was booked directly with A3 in Comfort Flex. I can't select seats for that flight either but on that one they have told me pre-seating is not available (or possibly not open yet but weren't able to advise if/when that might change). It's an ATR operated domestic service, and I'm assuming that seat selection will be possible at OLCI. Is it normal for those flights not to offer preselection?