If you want to make a draft, you can just make a new page. To do this in the Wiki, simply navigate to a page name that does not exist; the Wiki will then ask you if you want to create the page. (That's what I did with the page to monitor cleanup in Southwark).
If you want a draft for editing an existing page, I guess you could copy the contents to your new page, then make your changes there. If your changes meet with approval, you could then make the same changes to the "real" page, and then delete your draft page.
Or, of course, Wiki pages have an edit history (and don't we all wish WME did
). If you make changes on the "real" page and people don't like them, you can undo them.