Use a large c-clamp (I think the one i used was a 6 inch).

You can slightly loosen or remove the caliper mounting bolt first if you want, I don't think it will make a difference.

1. Unscrew the c-clamp all the way back.
2. Place the stationary end of the clamp on the caliper side
3. Place the head of the side that screws in, on the back side of the outboard brake pad and screw the shaft of the c-clamp until the gap is closed.
4. Continue to screw in the shaft until the piston is fully pushed in all the way.
5. Remove the c-clamp.

At this point you should be able to change the pads as you normally would.

This is the way I did it when my pads appeared too thick to install.

It seems that not all RX300 owners run into this problem.

I think it could stem from the fact that :

a. It may have to do with how far down the rear pads are worn down to before that metal bushing will slide and become a problem; and
b. they may have the problem, but if they compress the piston using the method (or similar one) described above they don't run into the issue.

Can some one who has replaced the rear pads write a step by step DYI.

I recall that only a center pin (locking mechanism) needs to be removed and the pads slide up and out. [There is no need to remove (partially or fully) the caliper assembly].

Also a step by step DYI for auxillary (parking) brake shoe replacement.

Salim

Got it done...
Piece of cake..