Never used a powder...
The bulk of my experience has been Italian, not French. I've not been to culinary school, so if I haven't done it at work, I might not know it at all.
That said, I was thinking demi as I have done it when I did work in a French place, roasting then boiling veal bones, skimming and reducing. As for Espagnole, I was thinking more along the lines of using pan drippings.
The Beurre Blanc thing had me in a quandry as to where it fit in. Add a shakey foundation in French food, and you've got some room for errors.
