I made in error in my question. My client is filing head of household with one child who lived with him. So his filing status is HH and not MFS. So he should be eligible for the EIC. Can I still use the lookback to 2019 when he was MFJ?

yes, but I think you only use the earned income in 2019 of the person youre filing for in 2020....I know I read it in one of those FAQs on the IRS website...Im digging!

And the return will have to undergo "special processing:" adding months to the processing time.

Agree. Not worth it.

Let's not get hurt jumping to conclusions.  If the taxpayer has waited until September to file a return, he may not be in a hurry for a refund.  He might prefer the 3% interest from IRS if the refund is just going into a savings account anyway.  Or he might owe money and would prefer IRS take as long as they want to send the bill.  

From what I've seen, IRS is going to delay processing for months whatever he does.