Well.. as long as you are using common instruments... i think it is a good idea. The important point is the skill level. With professional ensembles... it is going to be extremely expensive. But if you write something that is playable by amateurs, you can get live performance at a reasonable cost.
If you combine a symphony orchestra, a jazz big band, a marching band, and a church choir, you can actually get something close to what you have in mind. String section, full blown brass section, a sax section, a large percussion section, piano, organ, harpsichord. Recorders should not be a problem, and you should be able to find a good flute and clarinet section. All of these ensembles are available in some major universities, and you don't have to pay the musicians. It is common that the university will held a concert event where each of these ensembles perform in turn. So why don't write a grand piece where all of them can perform together ??
As somebody has pointed out, concert halls generally has a real organ. You can also use a church for performance. You can fit a very large orchestra in a church. The only problem with that is you may no longer have enough room for the audience :D
I would imagine that the piece should be very long, otherwise there would be no space to explore all of those different sections. Maybe some of its movements can be performed individually with smaller-scale orchestra, thus creating more opportunity for live performance.
So write those small movements first to get a basic idea of orchestration. Then you can proceed to more complex movements, and finally, write the grand finale with a BIG tutti.