Since there is no official guidance on the effective availability requirement under 1.404(a)(4)-4(c), I'm not sure what degree of confidence there is - depends on how aggressive you are (or more to the point, how aggressive your client is willing to be). Your proposed parameters seem pretty safe to me, although the minimum value to have a SDBA could be an issue. If, for example, the minimum is say, $5,000, and 90% of the NHCE's have $5,000 or more in their accounts, then it seems "reasonable" that this would qualify. I would tell them, if they wish to pursue this, that they need ERISA counsel opinion, etc. - the usual CYA stuff.
And of course, the plan fiduciary must determine that the fees are prudent and reasonable, etc., etc., and a flat fee for small brokerage accounts might not qualify.
As you said, potentially a bad idea on many levels.