" NCAA rules state coaches can't run camps more than 50 miles from their campuses, but it says nothing about "guest coaching" at programs run by other programs. "
Actually, the rules absolutely discuss being guest coaches at other camps. And it's explicitly permitted. Note that "noninstitutional" used below refers to "not the institution you work for"
220.127.116.11.3 Noninstitutional, Privately Owned Camps/Clinics—Bowl Subdivision Football.
[FBS] In bowl subdivision football, an institution’s coach or noncoaching staff member with responsibilities
specific to football may be employed (either on a salaried or volunteer basis) in any capacity (e.g.,
counselor, guest lecturer, consultant) in a noninstitutional, privately owned camp or clinic at any location,
provided the camp or clinic is operated in accordance with restrictions applicable to an institutional camp
or clinic (e.g., open to any and all entrants, no free or reduced admission to or employment of athletics