The problem is probably that even if you rewrite the username in 
Horde, the Sabre library that we use for CalDAV/CardDAV still sees the 
original user name and uses that for any generated URLs.
Since .well-known issus a full redirect, we don't know inside Horde 
that the user may be coming from there, so we cannot deal with it as a 
special case.
But can't you simply live with the fact that user@domain is used as 
long as the user is coming from and use the preauthenticate hook to 
always rewrite the username?

Horde "webmail" install working very well here
Horde Auth is LDAP, usernames are 'login' form (without @domain)

Configuring ca*dav with url works fine, discovers all 
calendars/addressbooks (/rpc/principals/username URL)

Now trying to use autodiscover feature (/.well-known/ca(l|rd)dav 302 
to /rpc/), using email address (simplest to use), I tried authusername 
hook, and even preauthenticate hook to change mail to login, Horde 
still send /rpc/principals/user@domain when authusername returned 
'username', and this finally fails to discover calendars/addressbooks 
(sabredav replies with "User user@domain does not exist").

I also configured activesync_get_autodiscover_username (which works 
well for activesync autodiscover).

Do I miss something ?

I'd love to propose a patch here, but I'm not confident enough with 
horde's auth code. Feel free to send me any pointer to show me the way?

I found an older ticket's comment 
https://bugs.horde.org/ticket/12305#c6 but I prefered open a new 
ticket? Hope I did the right thing.

Thanks a lot