you are correct, currently the only way to let a Miner authenticate via a web form is using a custom Miner. That's what has been done with the AUS-CERT plain text feed.
Adding a new Miner is not difficult at all, if you can send me more details (online or offline) we can work on a solution for your customer.
Basically the idea is creating a Miner supporting the web form, the customer then can just create an instance of the Miner and then provide the authentication credentials via WebUI. Obviously the credentials won't be stored inside the code or inside the prototype.
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