I´d imagine the leasing mechanic will be used with MRs also when they are added, given that usury was a sin, only when there was no risk involved while investment was allowed, the MRs could act as lenders in accordance to, the church law at the time, through kind of rental agreement where they borrow money to get a lease on a town/city for the duration of the loan(with possibly a chance for a ruler to either default or pay back the loan with forecasted returns at current income to get the town back on the tax base sooner). Could also play into how cities and later counties could fall to MRs due to loan defaults (CB for forced long term lease, possibly re-enforcable upon expiration or permanent) with first a city becoming defacto,kind of, part of MR and then opening up a way to force the rest of the county to join MR,through war, when the divided loyalties have existed long enough.
So in short, I would like to see loaning MRs with leased cities(same way as Holy orders/mechanic), Possibility of forcing a lease if there is default/there should also be a chance for continuing the loan if the ruler doesn´t have the ability to repay now, long term leases should come with a possibility of the MR gaining a CB on the county the leased city is let´s say after 50 years. Ofcourse this should be a independent ruler case to avoid vassals leasing their towns all the time to MRs, as per feudal rules the whole country belongs to the ruler from whom the vassals get the right to use a portion of it.
Also Vikings could be using this leasing system as they did establish bases along their raiding routes and ended up establishing the Danelag in England for example, could be tied to a "truce/don´t raid me please" as the Christian kings in England and around the Christian world were willing to be pay for not needing to fight the raiders, which is how Normandy came into being as the weak king of France gave Rolf land in exchange for protection against raiders. Coming with an threat of losing the whole county in war as the vikings establish their power on the land, ofcourse again through war, so you could buy short-term peace for a long-term disaster.
Anyways leasing system has many possibilities in my opinion for depicting division of ownership within a county, while still keeping them intact.