Is there an exemption that covers this model?

No. All three commonly cited exemptions fail on their own elements. The payment processor exemption requires operating through a clearance and settlement system that admits only BSA-regulated financial institutions — a public blockchain admits anyone. The “integral service” exemption requires transmission to be integral to a different service being sold — in a payments product, transmission is the service. The network operator exemption is reserved for EFT networks and clearinghouses between regulated institutions. (31 C.F.R. § 1010.100(ff)(5)(ii); FIN-2013-R002; FIN-2014-R011)