I was happy to see that a RDF/XML download is offered for every page. Is there any possibility to download all RDF data of a guide in one file?

not quite, but for any given openguides you should be able to get a 'scutterplan' by trying, e.g.

http://london.openguides.org/index.cgi?action=index;format=rdf

and that will give you a list of rdfs:seeAlso links that you can traverse to get all the RDF/XML pages.

there's no 'body content' in them though, only the metadata. don't know if there are plans or needs to change this...

zx