That does not work because $module_result.content is generated before the pagelayout is processed. Hence your variable $myvar is simply not defined yet in your override for node/view/full.tpl The other way around should work though.
I did the stuff in a different way according to your answer. I've defined and set as global a variable in the template which override node/view/full.tpl.
Unfortunatly I can not access this global variable from the pagelayout.tpl. Any idea ?