I can imagine several common dropbox-like accounts, upload-only for donating participants. An automatic notification email containing the link to the newly uploaded image which triggers an agent on the recipient's computer to start downloading for later stacking and processing.
If the given time frame were 1 week and the number of participants were, say, 150 per DSO/specified region of the sky (depending on the imaging goal to be discussed and decided in the committee
) - each recipient/admin of a dropbox would only have to manually handle 150 images (or the frames of 150 participants) per week (or per time frame set out in the committee).
Not too daunting a task for an individual, is it?
The next week, it'll be a different goal and a different group of stacking volunteers.
And the end results by each stacking volunteer can go up on Astrobin to a shared project account. To be used later for something else, like an all-sky-web-app Nick mentioned, or a scientific comparison with Hubble's successor, that imaging satellite, or something. Depends on the committee's vision.
