I found GetWorldInfo()._SetLayer() and GetWorldInfo()._SetZIndex(), but I was hoping there was an officially supported way.
Also, I'm not sure how to get _SetZIndex() to do anything. Do I need to separately splice into layer instance arrays or anything like that? (Edit: it does work with splicing)
I also see sprites still have the changeAnimFrame variable, but don't have C2 runtime's .doChangeAnimFrame() function. Is there an equivalent? (Edit: there is, as Erin pointed out below: _DoChangeAnimFrame())