Sorry Man, just searched through Coinbase - This is a direct copy from their website/support:
"Altcoins and ICOs
Jun 27, 2017
DO NOT send any unsupported digital currencies to your Coinbase account or participate in an ICO using your Coinbase account. Doing so will result in a loss of the digital currency in question.
Unsupported Digital Currencies
Because of the irreversible nature of digital currencies, the security and integrity of our private keys is of the utmost importance to us. Consequently, private keys associated with a specific address are virtually inaccessible outside of extremely limited circumstances.
As a result, Coinbase is incompatible with any digital currencies or tokens other than those found here.
Any unsupported digital currencies or tokens sent to a Coinbase address will effectively be lost."
If somehow you get a private Key… Or hopefully one day they will support MAID one day…
(Not sure if this will work - Try at your own risk!) Would he need to create a .json file to import to Omni? If you have the private key, you might be able to code your own .json file and import to Omni…
Might try to open a wallet with Omni…remove the address created from Omni…this should create a .json file. Then try switching the public key and private key in the .json code and see if you can import to Omni…