changing the display of formfeed characters

[Javier]

There are several suggested ways to display form feed (^L)

http://www.emacswiki.org/emacs/PageBreaks
http://www.emacswiki.org/emacs/PrettyControlL
https://www.emacswiki.org/emacs/pp-c-l.el
http://ergoemacs.org/emacs/modernization_formfeed.html

However all them break whitespace-mode.  Is there a way to display
formfeed ^L without interfering with whitespace-mode?

The issue has been known for long time.

http://lists.gnu.org/archive/html/help-gnu-emacs/2011-03/msg00891.html
https://www.emacswiki.org/emacs/pp-c-l.el

Has somebody come with any idea how to display ^L in some way without
breaking whitespace-mode?

Would it do the trick modifying the display table for ^L with a fixed width 
string?
Like "=========" and avoiding to dynamically adjust for window width.