If the company sees hiring as "tying up one of their Senior Devs for a day" rather than something so important that it warrants investing serious time/resources/Devs on it, then I don't think they "really want to hire the best people" at all.

I've been asked to work on a company's live code as part of an interview process, and they paid me pro rata based on the salary for which I was applying.

Asking someone to do so unpaid for more than an hour or two just isn't reasonable.