This has been discussed a few times, but things are different now that the 2A is incorporated.
IANAL, so I won't get into discussing what they can do legally.
Now, let's talk property rights. As a libertarian, I feel that property rights are just as important as the rest of them. I think if you and I want to write the 2A out of a lease contract, the state has no right to interfere. I should have the right to use my property in any way that I see fit. If you don't like my anti-2A lease contract, then don't sign it.
In a free market, those who choose to discriminate are only hurting themselves by reducing demand for their property. Some other entrepenuerial soul will see this as an opportunity to crush a competitor by meeting the demands of those who value their 2A rights.
The only time the government should be involved is for acts of agression. Surely, an optional lease contract is not an act of aggression. (See the Zero Aggression Principle.)