> This doesn't relate to get_params.

Yes, I got confused by the name.

> This relates to passing parameters into methods. It supports an old
> behaviour that allowed sub-estimator-specific kwargs and extends it to
> support universal kwargs, in order to allow Pipeline to be used as the
> base estimator of AdaBoost*.

Indeed. I see the problem. But when there is a problem in the API, the
solution is not to add a quick fix that is a hack, but rather to list the
problem on the API (this list is currently missing....).

If we try to solve everything with quick fixes adding a hack here and
there, we will quickly have unmaintainable code.