OK, so just booked some codeshare SQ along with AC Premium Econ to SIN. The AC confirmation has AC flight numbers for the SQ segments. I was able to pull up the SQ flights on the SQ app and was able to select seats in row 32 on their 777-300ER which is in fact the Premium Econ section. I feel reasonably comfortable and have an email that SQ sent me confirming the seat assignments and I subsequently emailed myself the AC booking that also notes the same seat assignments. I feel reasonably OK about things although I am also reasonably sure I won't win any arguments with SQ staff if they refuse to honour said seat assignments!!