You can use Opus with OpenCode anytime you want, just not with the Claude plan. You can use it via API with any provider, including Anthropic's API. You can use it with Github Copilot's plan. The only thing you can't do without getting banned is use OpenCode with one of Claude's plans.
If you want to use Opus with a different coding harness along with a coding plan, you can use Github CoPilot. It even has built in authentication with OpenCode.