The way I was originally thinking it would work:
1. The OP would post their track(s), and decide the BPM and the scale.
2. Everyone else would post their tracks in separate posts. Multiple guitar/bass/synth/whatever tracks would be accepted on each thread.
3. Once enough tracks are posted to allow creative composition, mixing would start.
4. People would download the tracks that they want to use in the composition and mix the song like normal.
It would be a very easy thread to post in like any other thread. Some would upload tracks while others are submitting mixes.
The thread would get messy and disorganized. There would be posts of tracks scattered within all of the mixes. When people are looking for the tracks to download, the would see all of the mixes and would have to filter though all of the pages in order to even begin to start mixing.
The other way the board could work is:
1. The OP would post their track(s), and decide the BPM and the scale. The OP would also state whether he would like to be contacted via Mixoff.org PMs or E-Mail
2. People would send their tracks (or links to their tracks (dropbox etc.)) to the OP according to his preferred contact info.
3. The OP would link the tracks in the first post making it easy to find the tracks to download.
4. Once enough tracks are sent to the OP to allow creative composition, mixing would start.
5. People would download the tracks that they want to use in the composition and mix the song like normal.
It would be tidy and organized. The only posts in the thread would be mixes. Just like it is with a normal song. The tracks would be right there for any person to get right on to the mixing.
The OP would have a big responsibility to continue to check his PMs, post the links, and such. If the OP no longer uses the forum, the thread would die quickly. So really, this way is very dependent on the OP.
In reality, neither one or the other has to be used exclusively. It could be stated in the OP's post whether he will maintain the thread or whether people will just add their tracks to the thread themselves.
Also, yes I think there should be a specific board for these types of mixes. They don't need to be organized by genre. Just one board named "Mixoffs Gone Wild" or "Collaboration Mixoffs" would suffice.
Even though it may be quite complicated, I would create a board and see what happens. It could be a total disaster or lead to the most fun forum known to man.