[exim] Case-(In)sensitive Plus Addressing with Maildir Deliv…

I've begun testing "plus addressing" to support the delivery of email
such as "user+topic@???" to a user-created IMAP folder:

/var/mail/user/Maildir/.topic/new/

Here's the config:

local_router:
         driver = accept
         local_part_suffix = +*
         local_part_suffix_optional
         check_local_user
         router_home_directory = "/var/mail/$local_part"
         transport_home_directory = "/var/mail/$local_part"
         transport = local_transport

local_transport:
         driver = appendfile
         create_directory = false
         maildir_format = true
    ...
         directory = /var/mail/$local_part/Maildir${if exists{/var/ 
mail/$local_part/Maildir/.${substr_1:$local_part_suffix}}{/.$ 
{substr_1:$local_part_suffix}}{}}

It delivers to the "topic" Maildir only if the user has already
created it. Otherwise, it's delivered to their top-level Maildir.

However, I've hit a snag with the case-(in)sensitive nature of
addressing. Despite the RFC's, users are accustomed to the fact that
"user@???", "User@???" and "USER@???" will be
delivered to the same "user". Some systems and web-forms even
lowercase addresses. However, a message addressed to "user
+topic@???" doesn't match a directory named "Topic" on an ext3
filesystem.

Does anyone have a suggestion to tell Exim how to deliver to "Topic",
even if the $local_part_suffix is "topic"? We realize one solution
is to force all IMAP folders to be lower-case, but would prefer to
explore other options.

Thanks!

Rich