Never seen one so never worked on one after looking on net some floor mounted some wall mounted in 'Inwall 6ltr cistern,very smart and neat but a pain as you've found out.
If 'Inwall' and it backs on to another room is their a cupboard or panel in that other room giving access to cistern, which I have seen done for other makes, to allow complete flat tile of wall in bathroom. Other than that if the answer is NO then it's a dismantle unless you can see the water level in the cistern and adjust to higher fill level.
I fail to see the logic in boxing in anything without maintenance access it's like building a porch with a door so small it cannot be used. Or even painting oneself in a corner with no escape.
You could hold fire for another reply
Nobby
February 2009