As the title indicates. Of course the 276Cx does support Custom Waypoint Symbols. And, if I manually add my Custom Symbols in BC to the 276Cx with the appropriate naming convention (Custom 1, Custom 2, etc.), using them between BC and the 276Cx works fine.
I've reported this through the 276Cx product team who said they would pass it along. Hopefully the BaseCamp team will address this in an upcoming release.
I have quite few Custom Symbols and would really like to use the "Manage Custom Waypoint Symbols..." option in BaseCamp.