Hi Stéphane,
We currently have a bunch of patches to get cyrus-imap dav proxying working in a murder setup, including the creation of the initial mailbox.
You can find the patchset in this branch:
https://git.kolab.org/source/cyrus-imapd/history/dev%252Fkolab-3.6/Given you've fixed some METADATA related proxying issues, you may also find some patches in that area interesting, I don't see myself getting to properly upstreaming them anytime soon, so feel free to do what you like with it.
Cheers,
Christian
