I recently had a Phillipines based agent call me back from live chat as I was adding a stopover to a redemption ticket so there were some taxes to pay. I asked her then to add the infants and she did so without any issue and calculated the fare (10%) that matched the fare that the website would have charged for an infant on a cash booking. Was quite surprised it was so easy!

She missed 1 seat assignment (787 and I do not want bassinet) so I called in today (India call centre I think) who proceeded to tell me that the seatmap wasn't available (despite my OH + infant on another ticket being seated already) and that SQ didn't do waitlists! So it does appear that it is very YMMV with who you get and what they can do