Here is again a question concerning this lack of force in expressiveness that the preposition "à" can be reproached with. Semantically and particularized to the sole context mentioned, there is no nuance. The only difference is in the effectiveness of the communication: whereas in the case of "à" one can have to turn the sentence in their mind before registering what is really being said, and have to rely on context too, in the case of "pour" the idea conveyed is more readily apprehended, although, there is in that second choice nothing more than an amelioration over the first.
As far as goes a possible nuance that could be found in virtue of a particular context, I think the discussion in this recent answer of mine does apply in the present case also.
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