Hi,
Ngui comes with everything you need to do that without any work on your end. Have you checked the samples provided by Ngui, almost all buttons have scaling behaviors. Basically, you can configure the way it scales and when pretty much to achieve anything.
Have a closer look.
It may simply be an issue with z ordering, try to move the Ngui plane in front ( modifying the z value of the transform), editing the z index might also be an idea if it's blocked by other ngui components.
Bye,
Jena