Pilot DAR;
If an aircraft is operated in EASA land on a Public Transport CofA, (or Continued Airworthiness) then without an MEL any snag can ground the machine.
Also the UK CAA is not in the habit of issuing MELs to owners, I know, I've written 5 different ones for different commercial aircraft.
Admittedly an MEL fo a Citabria is going to be very simple it allows the owner/training organisation who use the aircraft to carry on flying knowing that in X number of days a snag must be fixed.
I was surprised recently when our CAA FOI informed me that although one aircraft we operate is used solley to fly the owner and his familly it runs on a Public Transport CAT and therefore needed a MEL. I didn't realise they were going to go this far, but hey, they obviously need to justify their salaries and amazing pension scheme.
SND