My guess is that they would only help. It shows that you have an interest in aviation beyond big money and great lifestyle. Or even better, it shows some affirmative action affiliation.
I wouldn't necessarily include a union though...not all managers are big fans. Besides, they will know what airline you came from so they will know what, if any, union affiliation you have.
An exception might be work on benign union projects...safety committee, pilot mentoring, HIMS, etc.