So I am building a very simple multi-class kit, basically a shadowdancer/mage using the QD_Multiclass library.
I was able to work around some of the limitations, I.E., Mage/Thieves ignore backstab settings you set for your class, but adding in the backstab as a spell effect works, and it gives you an extra memorization slot when it shouldn't, but you can remove the extra slot you get every level with another spell effect.
However, for some reason hiding in plain sight always fails. When I used EKEEPER to simply switch the kit to shadow dancer and left all the effects alone, it worked. Switching back once more had it fail.
So I'm assuming it is not something that can be added to other kits or classes, but perhaps someone else will know for sure?