Not sure why the customer wants this but they want to use the inside int of a PIX as a default gateway for users on one inside network, 192.168.x.x to redirect to another inside network 10.x.x.x, I.e. router on a stick kind of deal.
I don't think the PIX can do this.
However, it does take a static route:
inside 188.8.131.52 255.255.255.0 10.4.2.31 1 OTHER static
...again both networks are on the inside.
Is this even possible?