If an employer fears this and truly feels this way that should be a signal to look at what's going on in their workforce, where gaps exist, and how someone with an "overqualified" work background can help the company close those gaps. You get to evaluate someone without feeling slimy for asking interviewees to work for free, the employee gets valuable experience and increases their worth to you as an organization and it makes it easier to promote them or give them commensurate pay (which: if this is what it took to pay someone overqualified what they're worth to begin with, then you probably deserve to have that person leave).

Sorry if it sounds abrasive-coming from a long history of being tired of the shallow excuses employers have for disqualifying great candidates while simultaneously bemoaning a "talent shortage".