[sc00b4s7eve]

I've actually played around with this quite a bit but it got frustrating setting everything up after every reflash/update I did.
If I understood you correctly: you like storing the message bodies (this means emails & txts alike) on the Internal Storage instead of the program memory.
HKLM\System\Inbox\Settings\
String "AttachPath" (attachments)
String "PropertyPath" (Message body)

(Disclaimer: This Is just how I did it. PLEASE GOD SOME FIND A BETTER WAY! The actual messages still needed PIMbackup to reinsert them into the .vol database file.)

So, The first thing I would always do would be to go into Messaging and check off the option to store attachments on the storage card. This makes WM create an attachment folder in the Internal Storage. The problem is that the name is unique & WM just created it as "AttachPath" in the registry (ie "\Internal Storage\Inbox.mst29979681.1250487424\Mail Attachments\")
Then I would go in and add the reg entry string:"PropertyPath" with a similar name as attachments (ie "\Internal Storage\Inbox.mst29979681.1250487424\")

Yay! That seems totally worth it right.... Right?
Well there's the reg entries to mess at least. Good luck. My next step would be to see if PIMbackup used the new values when I went to restore, I haven't tried that yet.