A multi-accumulator (MA), like the belt, has the advantage that the encoding does not need to specify a destination. It can also often avoid specifying one source, but pays for that with move operations to load the accumulator. You suggest using each FU as an accumulator machine, which is fine for ALUs but some FUs have no result to accumulate. In addition encoding would like a power-of-two number of addresses, while FUs are rarely provisioned in power-of-two numbers. Lastly, even a use-once may still need to be saved: (a OP1 b) OP2 (c OP3 d) where all ops use the same FU. All this can be worked out, but which approach would have the better entropy in real codes would depend on details of the encodings and the workload.
A larger issue is hazards: the belt is SSA and there are no W-W, R-W, or W-R hazards, while an accumulator is updatable and so someone must deal with hazards.
Because the Mill transiently hold results at the FU outputs one can see it as a kind of MA in terms of routing. It is not MA encoded; we are willing to pay the cost of the address remapping to avoid hazards and have efficient spill.