May I add my two cents worth? I was, as you know if you've been following me for some time, a church organist/minister of music in several different churches over the years. They have had different practices regarding use of the instruments, and the practices are very much a matter of the individual church organist and, to a lesser extent in most cases, the vestry -- the lay people who set policy for the use of the physical facilities of the church (they are advised by the priest or pastor or minister, and the organist, but they have the final say in most cases -- there are exceptions).
In most cases, the organ is off limits except under the direct supervision of the church organist. I would not have had it any other way; there are simply too many ways that an organ can get out of whack, and the risk of finding out about it half an hour before a service is simply too great. That said, I have always been able to use organs in other churches than my own, provided that I have gotten permission from the organist first, scheduled when I wanted to use it (and checked to see that there was nothing else scheduled), and made a suitable contribution for the care of the instrument (often simply being available to play services on request -- funerals, in particular, are hard to predict and schedule for...). But, of course, I expect that that was because I was also in the business!
My own policy was to give permission for use of the organ if possible, but only after at least one introductory lesson on the instrument, and some discussion of procedure and all. I have to admit that there have been occasions -- only a very few -- on which I felt it necessary to decline.
Some churches lock the organ. My last church, of which I am still a member, does not (nor is the chancel locked) and, to date, we have not had difficulties -- but we are a very rural parish.
In the one church I worked for which did have a piano in the chancel, it was available to members of the congregation provided that they arranged for its use with the organist (me, at the time!) and the priest; that was a vestry policy and I believe that it is still the case. That was, however, in a suburban parish, and in that instance the church itself had to be locked, unfortunately, due to vandalism. I may say that having to lock the church up made us all very unhappy indeed; it is not what should be. But a few occasions of arriving to prepare for a service and finding all in disarray...
There is a very difficult balance to be made regarding the use of a piano or an organ in the chancel of a church, particularly one which is open 24/7 as most of mine have been, for personal worship if not for services or service preparation (that is, rehearsing/practicing for the organist and the choir). It is not the same thing at all as a piano in, say, a school or college auditorium would be.
Just a few thoughts.
Topic: Church pianos/availability