I've got a patch for this. It pulls into a function all the initialization code that is dependent upon Dimensions or DOM properties, and then reinvokes that function when a drag begins. So if the element was not part of the DOM at creation (and hence never had its parentSize/axis initialized), it will pick it up on the first mouse action. It also deletes one call to offset() whose result is never being used.
As a nice (untested) side-effect, this should allow you to resize slider containers between actions without throwing off the slider calculations.
It still does not let you specify a startVal for a slider that's not part of the DOM - there's no way it could, really, since it has no dimensions to calculate a position and there's no event to hook code into between initialization and visibility. If my patch for bug #1737 is accepted, a workaround might be to call sliderMoveTo once the slider has been added to the DOM.
It also has problems with vertical sliders, but those existed in the original version, and so should be considered a separate bug.