Huh, that's an interesting idea. But as an engineer at a small automation integrator, who never has time between busy projects for important-but-not-urgent tasks like interviewing new hires, it seems I should be in your target market, but I can't imagine delegating a critical decision like this to a third party.
Who is your ideal customer? I'm imagining either a nontechnical solo founder (with a really good, high value idea that he'll explain after you sign an NDA) looking to hire a first engineer/technical cofounder to actually build it...and all the problems that come from that. Or I'm imagining a big corp with an HR department that's at odds with the engineering department, always denying their hand-picked candidates and sending them unqualified candidates, but I'm fortunate to have never experienced that kind of environment.
Maybe we're not in your target market in that we expect to need to do training, and don't expect our best long-term candidates to be able to hit the ground running at full speed. We've hired senior engineers with zero experience in our Autodesk Inventor CAD suite, as well as fresh grads with little experience whatsoever? Our "HR Department" is really just two people (our CEO and accountant), so there's no "must know how to lay out a robot cell" to validate in an assessment.
Or maybe we're not in your target market because already have an engineering department with something like a century of combined experience who are totally capable of sorting out a good candidate from someone blowing BS.
Or maybe we're not in your target market because we are just an ordinary small business, we have negligible turnover (it's been 6 years since someone moved to a different company), and only moderate growth rates (only hired 3 engineers in the past 6 years), it's just not that big of a time sink. I suppose a hardware startup with meteoric growth rates would need to spend a lot more time hiring engineers.
One question: You write "We can use any CAD software you prefer." Any? Really? Seems you need a short list here. If you're providing the license, having seats of Inventor, Solidworks, Catia, Creo, NX, Fusion, etc. on hand for occasional interviews sounds really expensive. And an experienced designer who can effectively every one of those is really rare and also expensive. I would call Altium/Cadence/Kicad "hardware design CAD packages", but those are completely different skillsets and I wouldn't expect a typical ME to know how to use them well.