Thanks all for your replies. The 4-wheel alignment shows OSR toe-out -0˚38' and the rear total toe -0˚33', whereas both NSR & OSR are supposed to toe-in. So, the alignment figures suggest that the rear axle isn't completely straight and there's no means of adjustment of course, because it's a fixed axle. Alignment figures are from gauges fixed to the wheels when stationery on the ramp, so this would suggest that the tyres can't be the simple cause of the problem (wish they were...) That said, there's no indication that the tyres are directional, so I could try swapping rear wheels to see if a different configuration might at least compensate/reduce the effect to some extent.
For information, these are the Alignment specs I was referred to:
MALAYSIA: TOYOTA ALPHARD MPV(2002 - 2007)
Parameter MIN MAX
Front Total Toe -0.11 0.11
FL Toe -0.06 0.06
FR Toe -0.06 0.06
FL Camber -0.41 1.25
FR Camber -0.41 1.25
Rear Total Toe 0.1 0.26
RL Toe 0.05 0.13
RR Toe 0.05 0.13
RL Camber -1.5 -0.58
RR Camber -1.5 -0.58
Left Caster 2.08 3.58
Right Caster 2.08 3.58