Hello,
although it is a practical impossibility to list all things "officially supported" by Alfresco (since there are so much variants of how one may use it), I can say from experience that
a) this is supported
b) anything else, which extends from an Alfresco base type like "cm:folder", "cm:content" or "cm:cmobject" should be considered as supported, as long as you don't mess around with any of the Java services (i.e. modify the source and rebuild them)
Of course, Alfresco Engineers are free to prove me wrong…
Regards