Does anyone have any insight on the approval process for prior service getting the new SSI-MOHC?

Just to be clear, the situation you are describing is that you met the criteria for a SSI-MOHC based on your service in the Marines and want to know if you can wear the USMC unit patch on your Army uniform. Is that correct?

If so, then I assume you're asking about the authorization route you would go through as mentioned in the regulation. However, I believe your ask is moot - the way the regulation reads, you're not allowed.

AR 670-1, Para 21-18:
b. Authorization. The CSA authorizes wearing the SSI–MOHC.
(1) Personnel who served in a designated area as a civilian or a member of another Service, but were not a member of the U.S. Army during one of the specified periods are not authorized to wear the SSI–MOHC.
(2) Unless otherwise approved by this regulation, Soldiers attached or under operational control (OPCON) to other services are not authorized to wear their patches as their SSI–MOHC without written approval from DCS, G–1 or a designated representative.

What you're asking (I assume) is in reference to (2). That section is for an Army Soldier that is serving under the control of another service.

The applicable portion to you is actually (1) - You were a member of another Service that met the Army's eligibility for a SSI-MOHC.

You can push for it by going to the beginning of the regulation and following the process under "proponent and exception authority". The address it would be sent to is listed there as well (under "supplementation"), but I don't think you'll really have any luck (worse that can happen is you get told 'no' though).

That is just the approval level that would be needed to approve it. Those are not approved because they don't meet the intent of the SSI. You will see a lot of people wearing them when you join the Army. They are all unauthorized. You will still see them, just realize that some SGM or CSM is going to call you out on it if you join the group wearing them. As SGM (Join to see) pointed out, there are only a handful of people in the Army who were ever authorized to wear them. Most of those people are SGM's or out of the Army now. However, you are still authorized to wear your CAR, if you have one.

To answer the question about "has anyone gotten approval"?
I've been in since 1993, served with a lot of former Marines.
I've never seen one get approval for service while still in the Marines.

The only MAR DIV patches you see in the Army that are authorized are just over a hundred from the Gulf War, and maybe a couple hundred from OIF (Fallujah 1 and 2), and all for Army service members who were OPCON to a Marine unit.

The DA is pretty stringent about this being an Army award for Army service members.