I have a wallet on Luno and another on Poloniex. Both pre-exist the Bitcoin (BTC) to Bitcoin Cash (BCH) fork. After the fork, Poloniex created another wallet for me containing the same number of BCH as I had BTC at the time of the fork. All good so far. Now I wished to transfer the BTC from Poloniex back to Luno, but I mistakenly sent from my BCH Poloniex wallet instead of my BTC wallet. Both Poloniex and Luno claim that the BCH that I sent to the BTC wallet on Luno is "lost". However, from my (very limited) understanding of the blockchain and the fork, would the BTC address on Luno not have been duplicated to the BCH blockchain at the time of the fork, and therefore be a valid BCH address? If so, how would I go about claiming that address - or can Luno do this since they still control that same address on the BTC blockchain and have the private keys, and would/might/(should??) be able to lay claim to the coin in the BCH blockchain?
In principle, Luno can claim these coins, but you cannot.
There is no distinction between BTC and BCH addresses. They each use addresses generated from private keys in exactly the same way. Money sent to an address, on either chain, can be spent by whoever holds the corresponding private key, and by nobody else.
So Luno created an address (call it 123abc) which means that they hold the corresponding private key (call it 567fgh). It's immaterial whether they created the address with the intention of using it for BTC or BCH (or both), or whether they created it before or after the fork, or whether it has been used before or after the fork (on either chain).
You sent BCH coins to 123abc (i.e. with a BCH transaction on the BCH chain), and so Luno is capable of using their private key to spend those BCH coins (again on the BCH chain), if they want to. Of course, they can also choose not to spend them, if they prefer not to, or find it too much trouble to adapt their software to do it. And even if they do choose to use them, you have no way (other than legal action or something similar) to force them to send those coins back to you, or to give you any sort of credit in exchange.
Unless of course they deleted the private key. If so, then the coins would indeed be lost, but the same would be true of any BTC coins sent to that address.