Thanks all - mixed news from my investigation.
CAAS have confirmed that any CRS conforming to FAA Technical Standard Order TSO-C 100b is approved for use on SG registered aircraft (CARES does).
However, SQ advise that "
since the harness crosses over the seat, in order to avoid inconvenience to other passengers we need to accommodate customers who need to use the harness in the last row of the aircraft".
Not sure how they figure it will inconvenience other pax as once installed, the harness sits between the seat back & tray table in the stowed position.
Anyway, don't see they'll accept that argument but it seems odd they put pax comfort over safety. I'll get off my soap box now and enjoy the holiday.
