To read flowed formated messages in Wanderlust put the following in your .wl file (you need flow-fill.el which included in the current emacs release):
;; Reading f=f (autoload 'fill-flowed "flow-fill") (add-hook 'mime-display-text/plain-hook (lambda () (when (string= "flowed" (cdr (assoc "format" (mime-content-type-parameters (mime-entity-content-type entity))))) (fill-flowed))))
If you want to write flowed formated messages:
(mime-edit-insert-tag "text" "plain" "; format=flowed")