Edit -> Cleanup?

I didn't know that option was there. I may have 2 1/2 years of experience with OpenMPT, but there are still several features I have yet to learn. No, my feature request is a little different.

Scrapping the second part of what I mentioned earlier, I think there should be a button next to each instrument or sample. Its purpose is to check if a sample is used in a song, and display back each of the patterns the sample is used in. This might be handy if one is making a large song with many instruments, and is unsure if he's used that instrument in the song. The Cleanup tool could then be used to remove all unused instruments if necessary.

But why would you want to have a button for each sample? It contradicts the use case you describe, "removing all unused instruments". If you are sure you want to keep a so far unused instrument, put a note in the pattern and it will no longer be removed.

After second consideration, as well as exploring the Cleanup option, I too have realized that this is a redundant option. Perhaps it sounded better in my head.