The newest version of mutt now incorporates the trash patch, so simply by upgrading to mutt 1.7.0 or later should give you the desired behavior.
From the ChangeLog for 1.7.0:
+ $trash, when set, specifies the path of the folder where mails
marked for deletion will be moved, instead of being irremediably
purged.
+ The function can be used to delete an entry and
bypass the trash folder.
I've been using the trash patch for a long time (and occasionally updating it to apply to new versions), so this is a welcome addition.