There are so many cool harnesses I could use if I didn't have an outrageously subsidized Claude subscription
This is what I'm doing too, and it's so much better. No more blocking copy/paste, no more stuck title bars, it actually sends notifications only when I need to give it input, it feels snappy, it doesn't destroy the scrollback buffer, messages are timestamped, I can copy my prompt out without spaces being added, it automatically bumps the agent to check when background processes run too long, and I added a sandbox that actually provides a modicum of security.
Do not worry about vendor lock-in. Standing against big corporations was never easier in this century.
Do it. You know you can.
At least that's what I'm using in https://beolis.com (so, you can use the actual harness you like and just have a proper UI to manage agents over it).
AFAIK - Of the big three only OpenAI explicitly allows third party harnesses without paying exorbitant API rates. It's one of the primary reasons I still use them.
To avoid these problems some folks use tools like herdr which operate like tmux or via the SDK to drive the official harness, rather than speaking directly to the API routes.
Whether and under what circumstances they actually enforce this these days, I do not know. Regardless, if your preferred harness does not support that authentication flow (which most don't), then it doesn't really help you even if you could. For what it's worth, I do not consider the Claude Code ACP adapter an adequate solution.