A) 'cron' is the wrong way to attack the problem. Use a procmail recipe
to detect HTML-formatted messages as received, and process them
at the time they are received.
B) There is a program called "demime" that will 'do the right thing'
for HTML messages -- if the message is multipart/alternative, it
will discard the 'text/html' part in favor of the 'text/plain' part;
if there is -only- an HTML component, it uses the 'lynx' web browser,
in non-interactive mode, to 'render' the incoming message into formatted
_plain-text_ and saves *that* in the user's inbox.
I have used demime -- invoked from procmail -- for this purpose, for years.