Yes - but don't be confused - vocals still only counts as one instrument where you can have 1-3 people singing. So technically you can have 3 people in a room playing vocals online with 3 other people playing their respective instruments in different locations. That's the only way to replicate the full 6-person setup.
After a song you can press a button (back/select I think) to get a detailed vocal breakdown, and for each mic it actually shows how much of each vocal track was sung. So you can try to stick with a single vocal role, then check the detailed vocal breakdown after the song to see how well you did! This is almost like the "locking into a set role" that you spoke of (you just have to use the honor system and it won't affect scoring), unless I misunderstood what you meant!
Yep, I've used that feature; it would be more interesting if, again, three people were cooperating. My rambling is just an uninformed brainstorm on how we might be able to get online harmony by using the dueling system...by adding another voice and controller. Ultimately, only a HMX developer knows whether this can be done or not.