[Joel]

Stencils don't work in the manner that you want, they're really just objects on special read-only documents that can be dragged to a canvas.

We do have it in mind to look at implementing some sort of reference object library in the future.

You might be able to get what you're after here with copying a shape as a PDF and using LinkBack to get the behavior sought.

That all said, the Selection Inspector can quickly filter and select similar objects throughout a document, and then you can apply style changes en masse.