Claude can be told to run OpenCode or other harnesses via CLI.

I've been doing this myself, all the time. (The ability to get second opinions and code reviews from Sol and other models is priceless.)

There's nothing restricting it to only controlling subagents that are other Claude models like Opus and Sonnet.

Also, I'm not sure that there's a one-level downgrade. Subagents can be pinned to any model:

https://code.claude.com/docs/en/sub-agents