Custom widget management

Is there a way to create custom widgets without directly editing /usr/lib/python3/dist-packages/qtvcp/ items?

I'd prefer to store these custom widgets in user directory to track modifications easier.  Otherwise I have to edit as root.

I've tried and failed to pinpoint where the include directory is defined.

Except for versa probe and basic probe, the short answer is no general way to customize widgets.
Versa probe and basic probe look for customized ui and handler files.
This describes the process for them:
linuxcnc.org/docs/devel/html/gui/qtdrago..._probe_screen_widget

You can always add your own widget to qtdesigner.
the plugin link should be in a hidden file in your home folder:
.designer/plugins/python/Link to qtvcp_plugin.py
you could add more links, but it's a fair amount of work.

What widgets do you wish to customize?

What I've done is added to qtvcp_plugin.py and in ~/.designer/plugins/python/user_plugins.py and in qt_handler.py at top It works ok, just one root mode needed that way