I wonder if a particular fedimint federation could enforce such a rule. The federation wouldn't care who I am, but it would be able to follow a rule and not let me spend my final 400,000 sats if it is locked up for escrow. In that case, you're already trusting the federation to not rug you, this just gives the federation one extra role. Not sure if that would be possible though. As long as the federation used by the merchant, and the federation used by the customer shared a linkage on a web of trust, the purchase could be insured.