I had originally asked this question back in the old REAL beta 0.5 days, around January.
In The Compositions Widgets folder "Presentation" lightbox or slideshow, Muse DOES allow for a Thumbnail Event trigger "On Rollover" option (see screen shot below). However there is no option for this in any of the Slideshow widgets, - only "click".
I still have one slideshow (link below), created about 7 months ago using the "Presentation - Rollover" method. It took forever & forever to do, because there is no simple "add images" folder option in the Presentation Widget.
Could you please add this feature to the final V2 release, since it's already over in the Compositions area???
Ron O. Vermeulen