Section 2.2 of the SEP article on modal epistemology differentiates possibility-first from necessity-first systems. Per modal logic, one can take these as metaphysical readings of the order-of-definition for the diamond and box operators. By analogy, then, one can speak of deontic systems as permission-first or obligation-first, say.
Now, to my knowledge, we might bracket Aquinas' ethics in terms of two imperatives (I say "might" in part to indicate that such bracketing is amiss):
- A trivial imperative, "Do good and avoid evil." (The linked SEP article tries to paint a nontriviality picture, here, but the whole article is fairly presumptuous and so anyway, I think the picture is not well-painted.)
- An apex imperative, "Undergo the beatific vision."
However, for other reasons, (2) is not well-stated. For example, if the divine nature is the one that provides the visio beatifico, and that nature is exempt from any external (or perhaps even internal) commands, or is at least exempt from nontrivial commands (so to say), then in revising (2) we are motivated to try out, "Make oneself worthy of the vision" (2') instead. Then, for the same reason that the first formulation was wrong, but in a different direction, (2') needs to be recast as, "Accept the vision if the divine nature gives it to you" (2'').
Unfortunately, that's still wrong: once the divine nature has displayed itself to a creature so fully, "the deal is done" and there is nothing to accept or reject (for one cannot, while already undergoing the vision, reject it). So (2'') might have to be more like, "Accept the grace that will make you worthy of the vision." Now, in this case, we do have an obligation—but it is one effectively relative to a supererogatory condition for the divine nature, i.e. its decision to impart (impute!) grace to this or that agent. It is, if you will, a deduction of an obligation from a supererogation: since it would be supererogatorily good for the divine nature to impart grace, it is obligatory for possible recipients of grace to properly dispose themselves towards the prior, divine supererogation.
Does this mean that Aquinas' implicit deontic logic, modulo (if anachronistically) modern technical sensibilities, can be taken for a supererogation-first system? The SEP article on deontic logic mentions one researcher (one of the article's authors) having a way to take a "least that can be done" operator to inform our understanding of a "it is supererogatory that" operator, in an ensemble of more typical such operators, so I would think (though I don't exactly recall from earlier reading) that the overall such system might allow for starting from a supererogation operator and then formulating the rest of the operators as modifications thereof.