Besides tmp:/, I found another means for main/screensaver "interprocess" communication: roRegistry. Registry i think is better choice for fast messaging between the two, since all happens in RAM and there is no need to reg.flush() - one can do reg.write() and the other right away reg.read() and see the result. Let me know if there is issue with relying on that behavior.
I would also like to see the official documentation from Roku on implementing a screensaver if it is possible? I can't seem to find it anywhere online When I search for Roku stuff it doesn't seem to be official - more just interesting info eg http://simplenewz.com/2014-09-14/Mainstream/feed/28577