Dear Owen,
Thank you for your query.
No, sorry, I do not believe that is possible.
As far as I know, all possible custom user interface options are demonstrated by the ADN Revit API Labs Training Material:
https://github.com/ADN-DevTech/RevitTrainingMaterial
https://github.com/ADN-DevTech/RevitTrainingMaterial/tree/master/Labs/2_Revit_UI_API
Look at the documentation and lab instructions:
https://github.com/ADN-DevTech/RevitTrainingMaterial/tree/master/Labs/2_Revit_UI_API/DocsCS
As said, I do not believe what you are asking for is included.
Sorry about that.
I hope this helps.
Best regards,
Jeremy