Hi Walt,
That is currently not possible. When using ftrack's simple UI functionalities available via the event hub (https://help.ftrack.com/en/articles/1040465-actions#:~:text=Read more here.-,User interface,-When an action). Using this method, no logic can be sent to change the UI dynamically and no message is sent back for you to process until the form is submitted. This makes it impossible in the current system to create a dynamic UI that refreshes "on_change".
I've submitted a feature request on your behalf for consideration by the product team.
In the short term, depending on your objectives, implementing your tool with another UI framework (standalone app, external web app embedded in ftrack or not) may be a better solution if its requirements exceed current capabilities.
Cheers,
Patrick