An adjutorious arbitration; an archaic word but then again, in the law nothing is archaic. There has to be a good deal of good will and cooperation where the parties fail to provide for an arbitration clause, a dispute arises and they nevertheless agree to resolve their dispute amicably using ICC Arbitration. So “adjutorious” in the sense of “helping” them to resolve their dispute. Accessory or ancillary arbitration doesn’t work because such an arbitration is the principal one. It ’s not ancillary to anything except the contract–not the dispute–and if you could use “ancillary” you might as well use “additional”. “Sequacious” could also be used in the sense of “following” the contract, but this word is mostly used to describe a person. No reason it couldn’t be used here, it’s just a question of repeated use and acceptance, but you lose the euphony of alliteration. Perhaps someone could start looking at the Selden Society’s volumes, there might be a term from Law French. Which, of course, is not French at all.