This really relates to creating a library that works across both WPF and Silverlight. The WPF Toolkit includes an implementation of the VisualStateManager included with Silverlight, and if the VisualStateManager can do what the MultiTrigger in WPF does, that solves my problem.
I just found an article on MSDN Blogs discussing the use of VisualStateManager and Triggers, but I'm still not sure whether or not VisualStateManager can be used as a replacement for MultiTriggers.