Thanks for proposing this!
One variation on the prize scheme that might attract more participants (funders and maybe competitors --- but will motivate people differently) would be to have the cash not go to the winner but instead go to the winner's choice of reputable computer museum.
The Lisa community is pretty small, and it might be the case that one of the donors or even an organiser or cheerleader for the challenge (nb: I'm not speaking with you/sigma7 or anyone else specifically in mind here!) could be one of the winners. Some people might be reluctant to contribute if they were worried about a conflict arising there, especially folks who are unfamiliar with us. ("What if one of these lisalist2 strangers had a working build all along, and this was just a way to make some easy cash?")
The nominal benefit of giving a donation choice to the winner might seem like less of a conflict, and the person contributing to the pot of money also gets to feel good that the cash is going to a museum. This may encourage more people to participate as funders. (This is also one reason to restrict the winner's choice to computer museums --- presumably those are of interest to nearly everyone who might want to compete or fund, and funders can give freely without any worry that the winner might choose a charity that they find morally objectionable.)
I'm thinking too that a donation bounty might encourage more collaboration than a cash bounty. If a pile of money is on the line, then competing participants may not want to share secrets or tools so that they can maintain an advantage. If the prize is a donation choice, then a competition may stay friendlier and may go faster as participants may wish to try and get the cash to a museum (and get a working build) as soon as possible.
A donation bounty also has fine PR value I think, and while we're not trying to market ourselves per se, a "feel good" story like this may spread further than the story of a traditional bounty. A wider reach could attract more people into the Lisa community, and that could have all kinds of benefits: what if some hacker gets it done by sprucing up LisaEm's Workshop support in the process, for example?
Of course, the motivation of a small personal windfall will be absent with this scheme, and I don't think there's any shame if that's what drives you to take up this challenge!
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Moving away from the nature of the prize, we could also reward desirable modifications and improvements to the Office System, like ways to use different storage media, networking, fixing the Y1.996K bug, etc.
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Anyway, just some thoughts. Thanks again for drafting this!