I might be wrong, but usually the fact that the scheme involves money directly means nothing to law enforcement. The main concern is the fraudulent act in which people are deceived for the benefit of others. So, if you run a ponzi scheme pretending that it is not that, it is a criminal act, no matter if it involves USD, BTC, gold, postcards, kisses, etc.

So, probably, if one makes it very clear to each person participating what it is about and inform them clearly of the inherent risks and losses that participants may experience, it (at least) may not be considered a fraud.

(But, anyway, it will always depend on many other factors, being some of these factors, the involved jurisdiction, laws, peculiarities, amongst others.)