You should explore the Action Framework. Lots of things you’d expect to only be able to do in JS can be done declaratively.
Here you could add some actions to your save button that “Update Field on Row” - passing the value from the UI Only Formula field into the real Salesforce Field before the save action occurs.
You could also trigger that action from the model - in the “Actions” section of the model configuration. Every time the Formula Field is updated - its value could be passed to the Salesforce field.
There is a sample page built on the standard Opportunity / Line Item / Product data model that has a lot of these ideas built into it. You may find it helpful.
https://github.com/skuid/SamplePages/tree/master/Skuid_Techniques/Action_Fwk_Examples
