I used the word invalid to mean a similar scenario to the OP's on reddit: https://www.reddit.com/r/CoinBase/comments/9v2cx2/accidentally_purchased_bch_and_sent_to_btc_wallet/, as one of the replies there states: "if nobody's able to claim that balance in their coinbase wallet after 30 days, then coinbase returns the bch/btc/whatever currency the original tx sent back to the original wallet".
But if someone somehow manages to access the sent BCH on Coinbase then they will not return the funds.
That person actually found a solution to get his funds back and wrote about it there, although their problem was that they sent BCH on Coinbase to a BTC wallet, not the other way around (sending bch from a wallet to a btc address on Coinbase, what OP is trying to solve).