That's right. What I mainly have in mind is a vector of Q-learned values V and a scalarization function that combines them in some (probably non-linear) way. Note that in our technical work, the combination occurs during action selection, not during reward assignment and learning.
I guess whether one calls this "multi-objective RL" is semantic. Because objectives are combined during action selection, not during learning itself, I would not call it "single objective RL with a complicated objective". If you combined objectives during reward, then I could call it that.
re: your example of real-time control during hunger, I think yours is a pretty reasonable model. I haven't thought about homeostatic processes in this project (my upcoming paper is all about them!). Definitely am not suggesting that our particular implementation of "MORL" (if we can call it that) is the only or even the best sort of MORL. I'm just trying to get started on understanding it! I really like the way you put it. It makes me think that perhaps the brain is a sort of multi-objective decision-making system with no single combinatory mechanism at all except for the emergent winner of whatever kind of output happens in a particular context--that could plausibly be different depending on whether an action is moving limbs, talking, or mentally setting an intention for a long term plan.
Interesting. Is it fair to say that Mollick's system is relatively more "serial" with fewer parallelisms at the subcortical level, whereas you're proposing a system that's much more "parallel" because there are separate systems doing analogous things at each level? I think that parallel arrangement is probably the thing I've learned most personally from reading your work. Maybe I just hadn't thought about it because I focus too much on valuation and PFC decision-making stuff and don't look broadly enough at movement and other systems.
Apropos of nothing, is there any role for the visual cortex within your system?
I too am puzzled about why some people talk about "mPFC" and others talk about "vmPFC". I focus on "vmPFC", mostly because that's what people in my field talk about. "vmPFC" focuses much more on valuation systems. Theoretically I guess "mPFC" would also include the dorsomedial prefrontal cortex, which includes the anterior cingulate cortex, I guess some systems related to executive control, perhaps response inhibition (although that's usually quite lateral), perhaps abstract processing. Tends to be a bit of a decision-making homunculous of sorts :/ And then there's the ACC, whose role in various things is fairly well defined.
So maybe authors who talk about the mPFC aren't as concerned about distinguishing value processing from all those other things.