I think that a Nissan Service stamp would actually mean less in the case of the Zed.
We all know of the stories, like mounting wheels back to front, charging for non existing pollen filters, scratching bodywork, etc. etc.....
They should stick to what they know and service the Micras and Almeras and the odd Quashqai.
If you do decide to sell, just point out why you decided to go to an independant specialist with experience working on a Zed and I'm sure they'll understand.