Wine start is not quite what I had in mind. Each file type can have multiple "verbs" (such as open, edit, print..), and I'd like GNOME to give me the option to use any of the verbs that are available. Wine start is apparently hard-coded to use the "open" verb.
That may mean it's simple to modify it for my purposes, but there's a windows equivalent. Changing the way it works to deviate from Windows may not be a good idea.
As for converting unix/windows paths, that's what winepath is for.