[exim] IMAP APPEND as a local transport?

I am looking at ways to decouple my MTA and message-store functionality.
Currently I am delivering mail into maildirs using the Exim appendfile
transport, but would like to be able to have the mailstore potentially on
different machines.

One way to do this would be LMTP of course, but I was also thinking that
IMAP could be used directly for this without the need to introduce
another protocol and program to the mix.

I was thinking of creating an IMAP local transport for Exim using the
current LMTP transport as a shell to start with. It would simply log into
the IMAP account and do an APPEND command into the mailbox. Has anyone
considered this before? Are there any downsides I am not thinking of?

I have noted that the IMAP RFC (3501) contains the following:

> Note: The APPEND command is not used for message delivery,
> because it does not provide a mechanism to transfer [SMTP]
> envelope information.

But I am not sure what it is talking about. There is no SMTP information
that the appendfile transport is putting into the maildir files other
than what is in the Received: headers.

Thanks,
David