"I can't find any clear guidance on whether it would be subject to SE tax and input on the C instead."

It is Ongoing or one time? Was it a reward or for work performed? Was it from a school or learning institution or from an employer (and the person is misclassified)?

Have you looked through the IRS resources?

https://www.eitc.irs.gov/tax-preparer-toolkit/schedule-c-and-record-reconstruction-training/1099-mis...