Seems practical. They already have an army of content moderators responsible for millions of feeds. Having them spot-check several hundred or several thousand third-party clients once every month or two doesn't seem like a large increase in their workload.
I mean, that's a single full-time employee (or less). If there are 2000 third-party clients and the goal is to spot-check them once per month, this hypothetical full-time employee would need to check 80 of them per day. That's one spot check every six minutes for eight hours a day if they work 25 days a month.
Not exactly the most exciting work, but easily doable. And that doesn't need to exactly be a highly paid employee either. I believe the big social media companies offshore a lot of that grunt work to other countries.
So we're talking what, thirty or forty grand a year for Twitter to implement that?