It sounds like this periodicity idea should just be a separate argument? So maybe you could just have my_fn(y ~ x, periodicity = 24). This is somewhat similar to the idea of case weights, which we just decided could be done as a separate argument that would be passed down to your implementation function. A few notes about that are here https://github.com/tidymodels/hardhat/issues/15
I don't think you should need to modify the blueprint functions? If you still believe that you do, could you explain more about why you think that, and what specific modifications you'd be making?