I have to ask: If you have an Emphasis system, why don't you use WYG to pre-vis? I realize that with Emphasis you are stuck at Release 17 of WYG, but it is already an integrated solution and would require no further expenditure.
Expression Offline does not have any ability to communicate with a visualizer (WYG or otherwise). Emphasis Offline can be used to pre cue Express/ion shows with or without a WYG dongle. With a WYG Design dongle, however, you do get shaded views which would be most useful for pre-viz. Again; you are limited with Emphasis Offline to WYG Release 17.
Best,
John
"There is nothing either good or bad, but thinking makes it so." - William Shakespeare