Patch-Tag now writes the _darcs/prefs/email file for repositories with the owner’s email.
That means if you check out a public repo (without being a member with write access to shared) you can contribute patches back to the repo creator simply with the command
darcs send
This assumes you have sendmail configured for sending email from the command line. But if you don’t, no problem, you can output to a file and send as attachment. Every repo has instructions that will hopefully make this easy for newbies, for example one of mine.
Thanks for the users who suggested implementing this feature, now that it’s there it seemed obvious all along.
Enjoy 
Yay! Note that the instructions have a couple of typos: abot; dont’; prjoect.
I would avoid calling it “pushing over email” because pushing implies that the patches actually get applied in the remote repository. “Sending over email” is probably a more accurate formulation.
Also, you can use the -O flag to avoid choosing a name for the patch bundle (darcs send -O will create a patch bundle using the same sort of name as it would have created for its attachments).
Finally, one use case to think about in the future may be projects (such as Darcs and xmonad) whose workflow involves sending patches to a mailing list; or similarly users who want to use separate email addresses for separate projects. So perhaps in the future it would make sense to be able to override the default email with one you supply over a text box (presumably with some sort of sanitisation)
Thanks for adding this!
Hey Eric, thanks for pointing out the typos and making the wording suggestsions.
Did as you suggested, and having a look at the email override feature.
Looks good