imapc master login for shared mailboxes

[Mark Zealey]

Hi there,

I'm trying to set up shared mailboxes such that any access to the shared 
namespace like INBOX/shared/<sub-email-address> will take <current-user> 
and use imapc to log in with <sub-email-address>*<current-user> (ie the 
current user is the master account for imapc). I've set up like 
https://wiki2.dovecot.org/SharedMailboxes/ClusterSetup on centos8 latest 
dovecot build (a late 2.2 series version I believe.

However I'm running in to a couple of issues:

1) setting imapc_master_user = %u has %u as <sub-email-address> rather 
than <current-user>. I can't find any sort of variable substitution that 
will allow me to access the details of <current-user>. From looking at 
the logs it seems to do a userdb lookup for<sub-email-address> to check 
it exists but then the substitutions appear to be using these rather 
than the current user's details

2) In a similar vein, I believe imapc_password = %w (ie trying to just 
get current user logging in with the password that was specified when 
they initially logged in, but as a master user) is also not substituting 
correctly and is just a blank string

Is there any way around this so I don't have to worry about trying to do 
sharing based on the filesystem and the same process but rather just 
allow the main user to log in to any other account (in this case domain 
owner logging in to any accounts under their domain) ?

Thanks

Mark