Update — there *is* a relatively clear answer to this, in the FAQ:
tax-the-rich.eu/home#faq

> The criteria for defining an "ultra-rich" should vary from one EU country to another, due to the economic, fiscal and social differences between member states. In Belgium, for example, we propose that anyone with 1.25 million euros in assets in addition to their main home and business assets should qualify as "ultra-rich".

So there we are!