@davidfor : Thanks a lot for your help and your reply! I've just found out what the problem was. I even tried your Kobo Utilities but was unable to update to metadata. Or at least : my shelves didn't update / show up.
1. something I forgot : I renamed my custom metadata shelves to #shelvesss - also configured this in the configuration options of my device in Calibre this time...
2. sent the metadata with the Kobo Utilities and with the standard 'send to device' : didn't notice any change.
3. deleted one of my books on the Kobo - resent it and saw that this shelf appeared...
So : I've just reset my device and will be resending my library. I suspect it'll be fine this time
thanks again!
/dimi