There is a thread on FW about a person who got shut down at Schwab along with the rest of the family when depositing lots of free-after-rebate checks. Not the same situation, but obviously they do look for funny business. I don't know specifically if they take MO via mobile deposit.
I deposited about 4k into Schwab in December. Just tried again last night on the mobile app and it said that it couldn't read it or something and to retry taking the pics. Took pics of the MO about 10 times and it refused to go through so I just deposited it into BoA ATM.