I think AirBnB are very unlikely to get into any negotiations with a freeholder regarding installation of key sites on properties.
First question: does your lease allow you to sublet - or take in paying tenant? If so, then you are over first hurdle. If it says nothing and you are the leaseholder then you can have paying tenants/guests if you wish. If you are renting, even if your agency lease does not reference subletting, you should really check the master lease for the building to see what that says, as that will contain more terms and conditions and override anything the agency has given you.
Finally, I think it's unlikely that a freeholder will allow people to include keys to communal areas in key locks.....they would see it as a security risk. So you either engage with them, or install it out of sight somewhere, but easy enough for guests to find. You are taking a risk of course if found out.
Also - do some reasearch in to key locks - there are some simple ones that anyone can open and are not secure. And there are better ones too...