There's a lot of truth in what you write and I definetely see the problem.
One solution is to have the take home interview as the final step in the process - after having screened the candidate and discussed general technology/framework/concepts without coding.
When I did interviews, I found that just by discussing things like thread synchronization and let them explain the projects that they've worked on would weed out most of the candidates which weren't a fit with the position.
Then the take-home problem would make a lot more sense for both the employer and employee.
Or another method:
You take home a problem, implement it, then present the solution to multiple potential employers.
The problem would have to be unique (so that there are no ready made solutions online) and trusted by the participating employers.
And I think this is exactly what OP is doing, and that's why it's great.
At the same time, I prefer the idea of presenting previous open source work instead of having a group of potential employers agree on a project that they can all judge. One benefit is that the work is less arbitrary, potentially useful for others. Another is it's more likely to be unique than an assignment is. Finally, do I want to work for any of those other employers?
Applying to a group that way would seem odd to me; part of me thinks that choosing companies is important, but another thinks that there's some chance that that's just because I haven't tried it.
Which, of course, leads me to agree that the OP is great as an experiment - it might well be a better solution in many cases. The fact that, in the OP, it's opt-in is a big deal as well. Though I'm still very skeptical of take-home interviews in general.
One solution is to have the take home interview as the final step in the process - after having screened the candidate and discussed general technology/framework/concepts without coding.
When I did interviews, I found that just by discussing things like thread synchronization and let them explain the projects that they've worked on would weed out most of the candidates which weren't a fit with the position. Then the take-home problem would make a lot more sense for both the employer and employee.
Or another method: You take home a problem, implement it, then present the solution to multiple potential employers. The problem would have to be unique (so that there are no ready made solutions online) and trusted by the participating employers.
And I think this is exactly what OP is doing, and that's why it's great.