interesting loophole in the rules actually, since they are separate branches of the same source, you haven't technically worked on that branch at all yet...
Though in the spirit of the jam you have inside knowledge and all that, so yeah it feels like you shouldn't
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That would be a different jam design, though a good idea for sure. A custom signup sheet could work, but requires a lot of guaranteed commitment of jammers. The % of joined to submissions is sometimes as low as 10-20%. Imagine 4/5 of assigned days being "no shows", it just wouldn't work.
They knew games would branch multiple times (perhaps a lot, perhaps so much that the last day is just two or three base games all branched out).
They knew games would die off. This part hurts a bit for projects that were actually good, so I feel like a "revival" opportunity or a "recovery" day might work for future jams to address this. Like a "Dead Pool" of strong projects that were not continued, that could be continued on any future day (with the downside of being down a day of development)
maybe you're right... though something that I thought might have been interesting is if the games worked on were ambiguous. That's part of the fun in the original pass the game challenge series--seeing what a average game can turn into within a matter of hours. This is what I tried with Budget Doom.