Is there a sane way of passing the filehandle on to another form, like in a hidden form element?

No. Filehandles are (operating) system resources ...  

Is there an approved way of passing this data about? Or am I best of whacking the data in a temp file and just passing the filename along the chain.

Sure, the concept is called sessions ^cpan.

merlyn has a written gamut of articles on this type of subject => http://www.stonehenge.com/merlyn/