If I understood things correctly, they do have in common that they employ chaining of execution units: a result can be used as an operand to another instruction without going through the register file. (The Mill does not even have a register file)

Otherwise, they are completely different. The Flow processor is more like a SIMT GPU modified to run general-purpose code. The Mill is more like a DSP modified to run general-purpose code.

Don't we already do that with forwarding?