It isn't too difficult if you are familiar with javascript and Moo Tools, which we used to create the gallery effects.
probably the most difficult part would be figuring out a logical place in the desing to initiate, pause and such.
Really if you use milkbox over slimbox that wil pretty much do it, and if you were familiar with moo tools that might make sense to you... ;)
Milkbox would work well to make a slideshow from the popup, making a slideshow in the page would require a little more cusomization.
We'll definitely be adding these features in the next major release if not sooner for instance in a dot release of new designs.