Found this thread from Google. Thought I would help. I was motivated to get this feature working for the Byteball link with bitcoin for their giveaway. After some "hacking" of Coinbase's sign message feature I finally got it to work. It is pretty simple:

- Use chrome (other's would work, but instructions for chrome)
- Go to where you are asked to enter your message to sign.
- right click on the "Sign Message" button and click on inspect.
- look for the "<form" tag above the button element that is highlighted in the developer tools.
- on that tag you will see an attribute called "action" with a value of "/addresses/xxxxxxxxxxxx" (whatever the number is for you).
- simply double click that address to edit it and add "/sign" after the number to complete the action address.
- that's it. The form will work.

Stupid bug for a site like coinbase imho.

Enjoy!