What's to stop Germany from doing this?

[ForzaA]

If they succeed in starting the war before September 1938, you lose all the Czech cores. If they can start it before March 1938, you lose Anschluss, the Anschluss forces (twenty Divisions or so) and all the Austrian cores.

IIRC, just being at war with Luxemburg itself is enough to keep those events from firing, no?

..Nevertheless, the Allies declaring war on you, which is most likely going to happen significantly before the usual start of the war, isn't a good thing either, most likely (as they'll gain both the usual benefit of going to war AND lose the peacetime penalty in the process, as they're going to be at war)

You can do it, but I doubt the benefits outweigh the losses/risks.