Even before considering accessibility, just normal keyboard accepted behaviors for RTL users...
LEFT key should expand a tree node and RIGHT key should collapse it.
Also, SPACE toggles expanding/collapsing of a tree node (which has children nodes).
As for screen readers, as far as I know all the above (RTL behaviour) is the same for screen readers
It seems like we can pretty safely change the controls around for RTL and it would behave in a way users would expect. This effects 3.1 (the new AMD navigation) and 3.0 (the old YUI navigation). We will need two different patches.
Note that ARIA does not have a consideration for RTL, so this will be breaking out of spec. Which seems safe, however it will mean JAWS and other screen readers will inform the user of the incorrect controls (perhaps, although Nadav's comment indicates otherwise). Either way, we need to check this and make sure we understand the consequences fully before integrating this issue.