And thinking about that a little more, it seems like it would be “easy enough” to write a small app that would let you:
- choose an arbitrary sport type (e.g. “Golf”)
- define a list of custom fields
- set values for each field (e.g. number of makes from different putting ranges)
- export a .fit file direct to the intervals.icu API
- somehow embed the custom field values into the .fit file
- use intervals.icu custom field scripting to extract the values and set the fields
Then I could use my custom app during my disc golf workout and log the results directly to intervals.icu. Hmmm, seems like a fun project, maybe I’ll give it a shot! 
EDIT:
And then thinking a little more, and maybe shaking my head a little, I realize I’m seriously overcomplicating this!
Rather than build a custom app to score my disc golf training, I can just use intervals.icu directly. I can plan a disc golf (“Golf”) workout, then click “Mark Complete” to create a matching manual activity, then turn on whichever custom fields I need and edit their values during my training session. Not quite as user friendly as a custom app, but pretty close.
@cjensen would this help for your usage?