The issue is that ARC AGI 3 specifically forbids harnesses that humans get to use.

So what? Are you suggesting that an agent exhibiting genuine AGI will be tripped up by having to ingest json rather than rgb pixels? LLMs are largely trained on textual data so json is going to be much closer to whatever native is for them.

But by all means, give the agents access to an API that returns pixel data. However I fully expect that would reduce performance rather than increase it.