I'd forego projects in favor of reference checks done by your more skilled dev teammates.
Former teammates can tell other devs if someone can code, and they'd also send up any warning flags.
For hiring juniors, I've had luck hiring for aptitude rather than skill.
That said, if you want projects to work in that framework, I'd make them last in the process as freelance contract-to-hire projects.
This way you get their best work in a resource-constrained environment and you overcome some of the other commenters good points about folks who don't have the time/energy/money to commit to a large project. As a bonus, you solve a problem for your team!
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