Potentially over complicated if you can settle for the ‘Discourse way of doing things’, that is: edit the data within the Topic Meta area, and later only when the pencil icon is hit.
You only need to render an input box that’s correctly wired up and make sure you’ve serialized the value to the Topic View.
The PostRevisor code will do the magic of updating the custom field.
Look at the Locations plugin example. Even that is more than you need.
Make sure you run the Locations plugin too to see how that looks.