Suppose we take Vicary's quantum harmonic oscilator as a kind of "toy quantum field theory". Next, take the category of internal comonoids to not represent the background causal structure. We normally take the comonoids to be something like the cobordisms in TQFT. Instead, we take the internal comonoids to represent classical causal structure (where ordering is taken from the string diagrams in the category of internal comonoids). This classical causal structure can be understood as the ordering of events in your laboratory (turning knobs, flashing lights etc.). The background, or underlying causal structure, is taken as the string diagrams in the base monoidal category. The idea being that the underlying causal structure is quantum and the interface to it is via the string diagrams in the classical surface which we see as the category of internal comonids. Thus, looked at this way, a quantum field theory might be seen as an epistemic restriction on (quantum) causal structure. We map diagrams in the background, quantum causal structure into the classical interface and this is the concept of an epistemic restriction. I guess the question is, can we interpret quantum field theory as an epistemic restriction on causal structure?