I don't think there is a way to do it *per account*, but there was a way
to customize the pref you're looking at.
Look at the value for the pref mail.identity.id1.fcc_folder as a
reference for the format. It may not be the id# you want, but if you
change the id#, you'll find the format you want.
Then you can change mail.default_sendlater_uri to
mailbox://youremailaddress/Unsent%20Messages where "youremailaddress" is
dependant on the format noted earlier.
I don't know if it still works, but that's the way it used to work.
--
Chris Ilias <
http://ilias.ca>
Mailing list/Newsgroup moderator