I'm not sure this will attract the best candidates. For a Master's program, candidates from from three pools:
(1) Students who just finished undergrad and want additional specialization before entering the workforce
(2) Working professionals who want to return to school to gain additional skills or enter a new field
(3) Those who never found a job and are trying to wait out the market in school
Of these, only (1) and (3) likely have the time to commit to a MOOC. (2) could (and many people do this), but will always have their normal responsibilities taking priority.
The problem is that a MOOC is a huge time commitment. If it's the only way to get into a Master's program, you're taking a huge risk if you're already working and have responsibilities. The GRE/GMAT + an application + interview is reasonable to ask for something that's not guaranteed and likely has an acceptance rate of 10-20%. A three month time commitment isn't. This will simply exclude the most desireable and qualified group of students and limit the pool of applicants to those who had the free time to commit to it.
It's kinda like companies that require programming assignments prior to interviews. That tactic, while trendy and popular, tends to exclude the top 10% coders simply because they have better ways to spend their weekends and evenings and know it.