There is currently no access the the Key/Fill Input setting in the Colour Key page of an input. I know I can use a Mix input in that setting, but the implementation of a travelling matte during a Fade transition, for example, is not as smooth as using a mask input directly in that setting.
Perhaps I'm missing something in how I set up my triggers/shortcuts. If I want to change a travelling matte in the Key/Fill Input setting, I have to change both the Key/Fill Input setting AND the trigger that plays that matte on transitioning in the input. I can do the latter via a script, but not the former.

Here's a link to the sort of thing I'm talking about. There are two videos with luma key mattes, and I can switch back and forth between them as the mattes play during the transition, via triggers. What I'd like to be able to do is change the matte via a script, but since the Key/Fill Input setting in the Colour Key page for each video is not available via the API, it becomes a manual operation. This is not mission-critical stuff, but it seems like it wouldn't be too much effort to make the Key/Fill Input setting available via the API.