Since the point of the thing is the black catkins I can't see the point in preventing those from being produced. But it seems pretty unlikely that they won't sprout again later from stumps created in fall.

Since the catkins form on new wood it would be sensible to coppice after flowering. Then you wouldn't lose next years crop of bloom but you'd still be restricting the tree's growth and achieving a neat bush for the winter. They are vigorous enough to take it, or you could cut back by half or a third as a compromise.

I wouldn't cut a willow back part way, as you get a hydra effect.

I meant half or a third of the number of branches completely to the ground, not half to a third of the length of the branches. Sorry if I wasn't clear.