Per-user "Seen" flags with shared namespace stored as maildir?

I was wondering if the above was possible. I know that the "seen" flag is stored in the filename, but was wondering if anyone knew of a way to separate it out? I know this would strictly-speaking break maildir compatibility, but it would be extremely useful for shared folders.

I've been given the task of replacing the (extremely heavily used) shared folders functionality of our Exchange server and have almost everything working. I have authentication working against AD, shared folders, per-user subscriptions, "mail enabled folder" functionality and seem to have sorted all the other permissions issues I've been having. Apart from a nice easy-to-use front end for changing folder permissions this is the last thing I can think of that I need to look into before I can show this to my boss.

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