As per request, the mechanisms for retrieving the hashes of accepted and rejected transactions are not obvious.
As a possible solution, Igor suggests adding two functions into jp.co.soramitsu.iroha2.Extensions
fun VersionedRejectedTransaction.V1.hash(): ByteArray {
return this.rejectedTransaction
.payload
.let { TransactionPayload.encode(it) }
.hash()
}
fun VersionedValidTransaction.V1.hash(): ByteArray {
return this.validTransaction
.payload
.let { TransactionPayload.encode(it) }
.hash()
}