?? Does anyone know of a website
which lists (per composer-piece) performances
worldwide?

For example, which orchestras on the planet
are doing VW 5 next season ?

I am familiar with three of them.

One of the best I have seen for any composer is this one for Berlioz. They do a great job of it.(Disclamer: I do contribute to it sometimes.) However, you can't search by piece, but they do have it organized for you.

http://www.hberlioz.com/Concerts/performancesd1.htm


I have used this one for Mahler several times as well. It's exactly what you are looking for.

http://www.gustav-mahler.org/mahler/concerts-f-e.cfm

There is also one for Vaughan Williams that I have used. It's not as well done as the top two.
http://www.rvwsociety.com/concerts.html

If you're looking for pieces that are under copyright still, you can sometimes find that information on the website of whatever publisher holds the rights to that particular piece.

For example, I know that Boosey & Hawkes has this search function on their website:
http://www.boosey.com/pages/cr...ndar/perf_search.asp

Obviously this only works for pieces a publisher has rights to, but within that framework it's pretty useful.