Although this "just so" utility function is valid, it doesn't peek inside the skull - it's not useful as a model of humans.

It's a model of any computable agent. The point of a utility-based framework capable of modelling any agent is that it allows comparisons between agents of any type. Generality is sometimes a virtue. You can't easily compare the values of different creatures if you can't even model those values in the same framework.

The reason I wouldn't call all agents "utility maximizers" is because I want utility maximizers to have a certain causal structure - if you change the probability balance of two options and leave everything else equal, you want it to respond thus.

Well, you can define your terms however you like - if you explain what you are doing. "Utility" and "maximizer" are ordinary English words, though.

It seems to be impossible to act as though you don't have a utility function, (as was originally claimed) though. "Utility function" is a perfectly general concept which can be used to model any agent. There may be slightly more concise methods of modelling some agents - that seems to be roughly the concept that you are looking for.

So: it would be possible to say that an agent acts in a manner such that utility maximisation is not the most parsimonious explanation of its behaviour.