I don't know if this is a good idea but maybe the OP can try use the brainwallet for the job.

When you sign a message, follow the link above.
Just Click on the Bitcoin-QT button, copy-paste the address, message and signature, (be careful with the spaces!!!) and if you get the green label on top with the Message verified to be from <YourAddres> then you can just click on the Signed Message button and copy the finished message format from there.

And see how it looks like in the link below >