In the chord notation window, it would be extremely helpful to have a chord named "NC" to notate "no chord". This happens frequently in real music.

Yes, this has been requested before. You might want to put it in the BIAB Wishlist forum, though, where it will be seen where it needs to be.

For some interesting reading, take a look at /BB/pgshortc.txt where you will see that NC is supposed to produce C. but apparently has not been implemented.