It does not seem to be possible to give rights to subwiki users on the main wiki right now.
To workaround it you could use some scripting to bypass the right check. Something like the following:
{{velocity}}
{{html clean="false"}}
$xwiki.getDocumentAsAuthor('xwiki:Common.Userguide.WebHome').getRenderedContent(true)
{{/html}}
{{/velocity}}