The easiest way to make JEPA and LLM to work together is to map JEPA encoding into input embedding, or inject it to a residual stream - perhaps via cross-attention.

But I don't think you can train JEPA and LLM together: JEPA requires a different training regime. Way easier to take already-trained JEPA and integrate it into LLM. The later can be like 30 minute experiment AI agent can code for you.