I'll tell you my biggest concern with "appointment on private property":
I download all caches in the area that I intend to be in to my GPS. There is no description, no clues, nothing. I find my caches by going to the nearest cache waymark on my GPS, and then to the next, and so on.
If I were to download your cache and go there at some other time of the day, I would be "violating" your idea of appointment only. Aside from that, I would be concerned that I found myself on private property.
To contrast with any other cache with similar limitations, if another cache is only "available" during certain hours (such as a park open during certain hours, etc, as the reviewer suggests), the hours tend to be posted and I have an opportunity to learn of the hour restrictions. Or perhaps upon arrival I would observe other indications that the cache can or should be retrieved at certain times, such as the presence of reguarly scheduled little league games, or high traffic areas.
Likewise, it would also likely not be private property, or other property where I might find myself concerned that my actions may be construed as suspicious or tresspassing.
I applaud your creative intentions, but I do not feel it is within the current spirit of the sport.
p.s. As to the comments about the cache being on an island in a lake, I have actually been looking forward to encountering this situation when I show up for a cache. I would be able to get a better idea of the area, and then properly prepare for a return. The joy of this sport is that we can pick and choose our "battles".