...of course, ATC are quite entitled to refuse such a request.
but you don't seem to see 'system integrity' as a valid reason for denying the request. Where I work we have little impact on the 'CFMU' level adherence processes as aircraft are usually still on climb to cruising levels, except for RYR aircraft that file at 260 or 280...
We certainly don't check every aircraft for level adherence before giving away requested levels. It would be impossible to do so across the board particularly for flight originating on the West side of the Atlantic etc.
We have very limited information in our electronic copy of your flight plan due to data issues; most of the route beyond one or two points outside our FIR is removed; thus we have almost no way of checking RFLs further down the line. We tend to ask the next unit if it looks like you are requesting something significantly different, if they say yes then we go for it; but this in no way represents 'how it should be done'.
We don't have management that the Spaniards currently have either, I have never heard of a controller here being asked by a manager why they gave a level not filed. Of course there will be an incident at some stage that will change this, I'm sure...
fireflybob, the level adherence is becoming ANSP SOPS, if your company SOPS said don't exceed 270 IAS below FL150 would you accept an ATC request for 310IAS till 20 DME from touch, or would you say unable to exceed 270IAS below FL150; even if you have been doing that regularly for the last 20 years? Unfortunately the CFMU level adherence system is going to be subjected to much higher scrutiny and 'auditing' and there will be "please explain"(s) made from various ANSPs. Welcome to the new world.
However, the problem with Strategic flow, ie sector workload protection by numbers of aircraft filed is it doesn't measure complexity. I have worked sectors over the last 20 years with 35+ aircraft on frequency and felt under no pressure and others with only 4-5 aircraft and felt under the pump; because of what is going on. The CFMU level adherence only attempts to regulate traffic the reality I feel, is far from perfect; ten aircraft at non-filed levels may be no problem on one day yet on anther day (or hour) just one may be too much... It also doesn't consider tactical reality, weather / turbulence, airport delays or anything short notice. etc.
Because Spanish ATC are implying the rules doesn't mean they haven't got a clue, perhaps they are just playing by the rules imposed; don't like it, bad luck, file at your "real" optimum flight level and see what you get then. We all know that certain airlines do file to get around the current CFMU system, they are playing the game, if the answer to the game is "no" then that's just the way it is, even if there is 'no apparent reason', no point bitching about it, especially here; you rolled the dice, you rolled snake eyes, see you next leg.