I did the vocals on this with 8 or 9 others doing various tracks.
I was thinking that the tools could make this a smoother process. I'd be surprised if something similar to what I'm thinking of doesn't already exist. I'm thinking that the 'producer' would do the backing tracks as before, but could select an option to share a mix with other participants. That would then automatically appear in their DAW project as a track without manual export and import. They would send their tracks back by the same method. It would be something like a shared Dropbox folder where you all get any files placed there, but would need some tweaks so you just get the final version when it is ready. The files involved will be fairly large, but shouldn't take more than minutes to transfer in general.
A further level would be the ability for others to monitor the tracking as it happened so they can offer suggestions. Actually playing together 'live' over the internet has issues due to the inherent latency (see below), but we're all merrily doing near real-time audio these days via Skype and other services in very reasonable quality.
Ohm Studio offers most of these features, but is not available on Linux. It looks like they anticipated my thoughts. The MIDI should work well as the files are pretty small. It looks like a smooth experience.
It would be cool to see this sort of thing implemented within Ardour.
Another type of collaboration is the jam session. As mentioned above it's hard to get low latency due to the nature of the internet and the limits of the speed of light. Ninjam has been around for years and gets around this by using looped backing and not sharing a player's audio until the loop ends. I've tried it briefly by running Reaper on Wine, but didn't fully get the hang of it.
The standalone Ninjam client hasn't been updated in a few years, but I just found a project called Wahjam that has taken the code and seems to be in active development.
I also came across ejamming that seems to have found ways around the latency issue. It's also not on Linux,but looks like clever stuff.