If i understand your needs, I seen 2 ways (if anyone have another i'll thanks to seen it too)
You dont know the deep o site, but you know the level of top of your subnavigation. why don't use path too find the top level? something like this: fetch('content','list',hash(parent_node_id,$module_result.path[2].node_id))
The other ir create an ini file, define there the id of top subnavigation and use ezini in template. or define directly in template (i really don't like this solution).
I think that $node.path_array is an array of the parent nodes ids of the actual node. $node.path_array[0] if the id of root of the site $node.path_array[1] is the id of main folder.
You may use the following code to do what you are asking for.