Because of the MIAA transfer rule, will a school put a freshman on varsity just to lock him in?
Yes lots of schools do this.
Disclaimer: lots of schools do about 1 of these per year, every year.
Most MIAA varsity teams have 4-5 freshmen on the roster. Sometimes only 2-3. Another 15-20 freshmen on JV. Another 15 on Fresh/Soph.