Thanks for your comments and links!

The torx T40 seat post screw seems to be done up really tight.

I believe there are torx wrench sets available like Allen key (hex) wrenches, but the local hardware stores I visited do not have them!? Even if the stores had the torx wrench sets, I might not have purchased one because the leverage on these wrenches would probably be very limited due to short 'handle' length (maybe 4" long).

Using Vice Grip on the screwdriver did occur to me, but given the suspect (weakish) construction of the screwdriver, I thought it most likely that the screwdriver would be damaged in the process.

In the end I picked up a 3/8" socket with a torx T40 bit (similar to Craftsman) from a hardware store. And using a 3/8' socket wrench (with a 7 ~ 8" long handle), I was able to unscrew the torx screw and adjust the saddle position!
Although the socket wrench had more leverage, it was still a bit difficult to undo the screw. Whoever tightened the screw in the first place must have jumped up and down on a wrench!?