[Doug]

not by one person. But four could - each singing a different note of the 7th chord.

My teacher was telling me about secondary dominant chords whereby you can precede a Dominant (or any other chord in the harmonized scale, really) with "it's" dominant. So for an example in C of a IV V I - Fmaj7 G7 Cmaj7 you could introduce the V of the G before the G so you have IV V/V V I or in actual chords, Fmaj7 D7 G7 Cmaj7. The D is the V of G so it's a secondary dominant. And I think these secondary dominants can be played before any of the in-scale chords.