I have configured my model tree with some columns and sometimes it is hard to see if the part that I clicked is in the same line of the information displayed on a distant column, so it'd be nice to have an option to add horizontal lines between columns in Creo. I will put an image to clarify what I am saying.
Your post appears well documented but has not yet received any response. I am replying to raise awareness. Hopefully, another community member will be able to help.
Also, feel free to add any additional information you think might be relevant.
AFAIK, this is not possible using built in functionality. It may be possible using an API to modify the UI elements, but I suspect even there it is not supported.
One option to help with organizing the tree would be to add comment lines. This can be done for example by placing empty note annotations in your start parts to include markers for the model tree. It does not provide the visual reference you are seeking for horizontal alignment but might be useful for you. As shown below you can name these "markers" to facilitate organizing or understanding of design intent.
