Here is a "workaround". It takes some time and preparation, but it will serve as a working save draft mechanism in the interim.
Compose your draft, and e-mail it to yourself, making sure to enable the "Save to sent messages" function. Use "Draft" as the first word in the message title to make it easier to spot.
To continue work, highlight & copy the text, click "Compose", then paste the text to the composition field. Repeat the send & copy process for each draft modification, then give it the proper title and send it to the proper recipient when the message is all done.
If you are working on or keeping multiple drafts at once, you can also go to your "Folders" section, create a new folder, and name it "Drafts", and move the drafted messages from your "Sent" box over to it. They can then be copied & pasted to the compose window for further work.
An added bonus of this is that if you share your machine, it will allow you to conceal them from prying eyes or meddling fingers.