Can a Plan establish some parameters on who is eligible for self-directed brokerage accounts? Are there any standards out there that have been approved and are not considered discriminatory? Could a plan use the SEC definition of an "accredited investor" even though that is someone who has relatively high net worth? Or is the answer no to all of the above due to the current and effective availability tests for benefits, rights and features?

No, the plan cannot establish the minimums.

The only solution is offer a 'brokerage' account someplace that has its own minimums. That way the plan is offering but an outside party is setting the restrictions.

And even when the brokerage firm sets the minimums, you should test availability to HCEs and NHCEs for coverage just to be safe (I have a number fo clients that do this).