Tapestry's current behavior, with the 5.3 DTD (and the current 5.4 DTD) is that parameters are matched against any mixin, or the component, with a formal parameter.
A problem exists when a mixin has a formal parameter and the component (or other mixin) supports informal parameters; the mixin will "take" the unqualified attribute as a parameter.
I think a better solution would be to require that mixin parameter names be fully qualified, always.
Of course, there's more ambiguity in the Component annotations's parameters parameter.
I would suggest that parameters be added to the Mixin annotation, and it should be used inside a Mixins annotation.