You will have to get access to your private key in order for any of the other methods to work for you.
...There is no way to get the Mastercoin funds out of there without either a option to send multiple outputs in a transaction, getting the private key of your address or getting the public key and an option to sign a raw transaction at Coinbase.
Ok, from Coinbase's perspective, they'd have to implement these transaction classes for users, because they don't give users private keys.
I'll relay this to Olaf from Coinbase, who's been sympathetic, though unfortunately unable to get me out of this mess.
I think we should start a thread to address this issue. It would warn people about it, and help others who, like me, are victims of the decision to only support sendmany transactions - which was not made known until after the window period to initially buy Mastercoins came to an end.
To have supported this project and subsequently found that the Mastercoins I bought are locked ad infinitum is a horrible thing. Something should be done about it.
I'd just like to add that the problem here is NOT the sendmany requirement, but the requirement that you be able to send FROM a particular address, which coinbase doesn't give you. Even if coinbase added sendmany support, you would still be stuck.
People who invested from Android wallets (which don't support sendmany) have reason to be annoyed, but they can still get their MSC by exporting their private keys.
I've tried to make this requirement (to not use web wallets which don't give you access to private keys) clear any chance I get. I'm truly sorry that you invested without reading a bit more about MasterCoin's specific requirements.
Thanks, Dacoinminster and Tachikoma, for your responses.
Tachikoma, I've not known about the possibility of signing a raw transaction at Coinbase. I'll definitely try that one on Olaf, and report back.
Dacoinminster, I'm no expert and so I'll be the first to admit to being mistaken about what one shouldn't do with Mastercoin. However I did read the majority of this thread before 31 August to try to understand what I was doing, but, as it turns out, I ended up with the wrong idea.
I was clear that an Mt.Gox wallet wouldn't offer "full control" over my addresses and therefore wouldn't work. However a Blockchain wallet would work, even though it's a web wallet. Unfortunately I found no clear description of what "full control" constitutes, and so I made the mistake of assuming that because Coinbase supports address creation and retains a list of addresses used, that this was a sufficient level of control. This conclusion was based on the fact that Mt.Gox addresses associated with my account are not retained for me to access, whereas Coinbase addresses are.
So yes, I was mistaken. But I think that there was a significant lack of clarity about what would and wouldn't work. A simple list of wallets that wouldn't work would've sufficed. I spent several hours trawling this thread for clarity, but ended up being misled.