From what I have heard, FIRST will be handled similarly to marching band, dance, concert band, orchestra, choir, and drama; all of which are MSHSL-sanctioned activities.
Most of these are not as restricted as the MSHSL sports are. They are in a different class of MSHSL activities with different regulations. I am not sure exactly what the differences are, but I never felt as though the league hampered any of the other activities at all.
EDIT- You can find the fine arts rules and regulations
here on their website.