I am inclined to agree with you, I do remember getting a NotImpletmentedException when I initially tried to create the ClientFeatureDefinition, which may imply that they intended to add the functionality, but ultimately didn't.
These observations would also fit with this PowerPoint presentation I found deep in the forums from 2007, annoyingly I can't find the original post, where one of the last slides points out possible future enhancements.
Which leads me to my next point, what should we do now? if I put this forward as an idea to add the functionality, how likely is it that it will actually be considered?
Edit: I've found the original post, where @philippe.leefsma posted a zip file containing the PowerPoint I attached (creating custom feature - Autodesk Community - Inventor)
Here is the link to the idea, any support is greatly appreciated:
ClientFeature for Assemblies - Autodesk Community