Section 1. If a forcible entry into land or tenements has been made, if a peaceable entry has been made and the possession is unlawfully held by force, if the lessee of land or tenements or a person holding under him holds possession without right after the determination of a lease by its own limitation or by notice to quit or otherwise, or if a mortgage of land has been foreclosed by a sale under a power therein contained or otherwise, or if a person has acquired title to land or tenements by purchase, and the seller or any person holding under him refuses to surrender possession thereof to the buyer, or if a tax title has been foreclosed by decree of the land court, or if a purchaser, under a written agreement to purchase, is in possession of land or tenements beyond the date of the agreement without taking title to said land as called for by said agreement, the person entitled to the land or tenements may recover possession thereof under this chapter. A person in whose favor the land court has entered a decree for confirmation and registration of his title to land may in like manner recover possession thereof, except where the person in possession or any person under whom he claims has erected buildings or improvements on the land, and the land has been actually held and possessed by him or those under whom he claims for six years next before the date of said decree or was held at the date of said decree under a title which he had reason to believe good.
(M.G.L. ch. 239, s. 1.) The language here, while permissive (i.e., "may"... ), covers a broad range of situations that don't necessarily involve any transaction that could pass for what might be called "rent". So, for example, the first two cases cited would seem to cover pretty much any instance where a person was camped out on another's property claiming to be there lawfully. The longer underlined bit applies specifically to the case I was worried about.
The permissive nature of the statute is potentially interesting. The litany of cases seem to contemplate action by the person asserting rightful possession of the property, which leaves open the possibility that a landlord is not required to use summary process to remove a person on their property. They could alternatively just seek a restraining order. Also interesting is the apparent threshold requirement of a "decree for confirmation and registration of his title to land" by the land court. If this were taken seriously, almost no case would be ripe for the housing court or summary process since few of the litigants ever come with an explicit judgment from the landcourt that they are actually entitled to possession.
On the other hand, it seems that given the ability of litigants to invoke summary process even without having a land court decree, the courts must be interpreting the scope of summary process to apply to any litigant who could potentially turn out to be the rightful possessor. This is problematic since even if the court gives litigants the benefit of the doubt about rightful possession of title, the statutory language limits summary process to only those who have such title. As such, mere tenant (who, while they may have rightful possession, don't have rightful possession of title to land) does not appear to be within a class of people who may invoke summary process. In other words, were a "landlord" to opt to not pursue the ouster of a "tenant" in housing court, instead seeking a restraining order/injunction/etc., there would be no way for a tenant to force the case to be heard in housing court. A judge faced with a motion for such an injunction outside of the housing court ought then to deny any motion from a defendant seeking to remove or transfer the case to the housing court! This is not what usually happens, of course.