I don’t understand how the CustomObjectFrame will help. If we provide it any plugin (not just yours) may use and modify it.
If you really need a frame only you control then you should save a plane/xform/whatever in UserData like I outlined here:
Not currently. I opened RH-85938 to fix it. When a new object is added it usually copies its attributes from some other object. In case of commands like Join/Explode you can get some funky results copying frames from the input. I think we need to tune that up for when something is copied.