It can’t be discounted that discussions between the two sides could just be discussing another musical project, like recording a radio-friendly version of Part of Your World to go on the soundtrack of this film or something of that nature.
Although, Musgraves could just as easily end up recording a new song for the soundtrack in addition to playing a role in the film, even if the two things aren’t directly related.
Kacey Musgraves has worked with Disney in the past, most recently she recorded a version of “All is Found” for the Frozen II soundtrack, which could be heard over the film’s closing credits.