Re: RMAIL: output message into a buffer ?

[Richard Stallman]

    What I'd like to add (or see someone to add) is something that,
    on demand, would fetch the current (or more) message, insert it
    into a "temporary buffer" and undigestify it into this buffer
    leaving the original message untouched (thus preventing rmail to
    do extra work onto the original rmail file). When in this
    "temporary" buffer, rmail would way quickier since it would have
    to just undigest the message and recompute a summary based on a
    very small amount of data.

It sounds like a reasonable replacement for undigestify-rmail-message.
After you have the sub-messages in the other Rmail file, there could be
a second command which says to put them back into the first Rmail file
in place of the digest.