If i were in your shoes, i would call the Mass dept of medical mj, and ask to speak with someone. Ask that question. Be sure to get the name of the person you speak with. Report the response to your landlord.
Also, in many states, the owner/landlord is required to give 24 hours notice to the tenant before entering the property. I would ask your landlord to respect this law, and provide notice before showing the property. That way, you could at least be home for the showing.
i have a friend going through this right now but his landlord has made him remove his garden. the landlord gave him enough notice to time the harvest with the sale of the house. maybe you can ask if he could not show the attic until you have moved already. did he promise to sell to someone who is interested in continuing your lease? you might want to ask about the laws about permission to grow on his property because even though people might not have the right to enter, he may have the right to shut it down.