According to the SolidWorks knowledge base, this functionality isn't available:
SPR 369795 - Would like to have API support for the New 'Display State' functionality in SW 2008
What is strange is that this SPR is marked as "critical" and yet it hasn't been addressed in probably 6 years. I would suggest that you email API support to have your name added to the notification list for this SPR. This effectively acts as a vote for this enhancement.
Thanks Keith, I added myself to the notification list.