I might be wrong, but can't you checkpoint the post-system prompt model and restore from there, trading memory for compute? Or is that too much extra state?
My mental model is that the system prompt isn't one thing, and that seems even more apparent with line 6 telling the model what today's date is. I have no insider information but system prompts could undergo A/B testing just like any change, to find the optimal one for some population of users
Which is to say you wouldn't want to bake such a thing too deeply into a multi-terabyte bunch of floating points because it makes operating things harder
