Well I just described the legal limits or what would be legal.
As said before, the pilot in command or as he is called in JAR OPS the commander may decide to take extra fuel. So whenever you feel to take extra fuel you can do so. As the descirbed scenario is in my point of view a very valid reason to take extra fuel, I would do so.
It will get different, if you can't take any extra fuel, as there is no tank capacity left or you would exeed any mass or performance limits.
It of course also depends on the people working in dispatch doing the planning and finaly it also depends on the company's, let's name it culture in respect how to plan and run the flying show.
For example they could plan an aerodrome as destination alternate with two seperate runways, which requires the higher amount of fuel and so be taken as alternate fuel when caculating the minimum fuel. By the way, as a commander you can always request this, at least I can and always could do so. This would give you at least a last option and you are on the safe side. Dispatch can give you some kind of extra or additional fuel, however this might be called internally and finally they may also decide to hold your flight.
Like always we can't cover all possible scenarious, but if you keep comon sense and know what you may do and finally what you are responsible as commander according to JAR OPS, you will find a soulution to this problem. I know this is sometimes hard and you have to stand your ground in the evironment of penny counting people in management, but it is possible.