Assuming a 401k plan passes coverage, are resident aliens excludable from the plan? Is there any possibility this would violate any federal non-discrimination laws regarding discrimination based on race or national origin?

Is there no other classification that would create the same result?

I would be very concerned about discrimination based on national origin. I would also be concerned that the sponsor may not know each employees origin and make a mistake in calculating eligibility

There is no other classification that would exclude only these employees. There are US citizens who do the same job, so they cannot be excluded by job. The employer wants to exclude all individuals working for them on a green card regardless of nationality. All of their green card employees happen to be hispanic.

That sound like a pretty clear violation of discrimination laws to me.

P.S. Of course I'm not an attorney, so don't take my word for it.