Toyota apparently felt that having the rear sunroof open was a 
potential safety problem.  However, dual sunroofs are available 
in europe and elsewhere with the rear sunroof that can be popped 
open.  

You can modify your RAV4 by doing the following:
 - Obtain a replacement sunroof latch from Toyota 
   (P/N 63250-16020, dealer cost of $42.22, retail is $50-$60)
 - Replace the rear latch.  This is very easy, just remove 2 trim 
screws and 2 bolts that attach the existing latch, and reverse the 
procedure with the new part.
 - Remove the rear sunroof, and remove the latch mechanism from 
the roof with 2 screws.  You will need to disassemble the latch 
and reassemble the locking arms in reverse so that the sunroof 
will lock in place.  
 - The arms are held in place with snap rings that can be pried 
off with a small screwdriver.  Be careful!  The snap rings will 
fly across the room unless you cover them when you remove them.
 - Remove the latch arms and replace them in reverse, i.e., the 
left arm on the right, and vice versa.
 - After reassembling the latch arms, reattach the mechanism 
to the sunroof, and reinstall the roof.
 - You can examine the front sunroof latch system to see how 
it is supposed to look if you run into any difficulty.