Consider then a virus particle ... Surely there is nothing in biochemistry, genetics or other science which implies there is anything our very particle ought to do. It's true that we may think of it as having the goal to replicate itself, and consider it to have made a mistake if it replicates itself inaccurately, but these conceptions do not issue from science. Any sense in which it ought to do something, or is wrong or mistaken in acting in a given way, is surely purely metaphorical (no?).

No. The distinction between those viral behaviors that tend to contribute to the virus replicating and those viral behaviors that do not contribute does issue from science. It is not a metaphor to call actions that detract from reproduction "mistakes" on the part of the virus, any more than it is a metaphor to call certain kinds of chemical reactions "exothermic". There is no 'open question' issue here - "mistake", like "exothermic", does not have any prior metaphysical meaning. We are free to define it as we wish, naturalistically.

So much for the practical ought, the version of ought for which ought not is called a mistake because it generates consequences contrary to the agent's interests. What about the moral ought, the version of ought for which ought not is called wrong? Can we also define this kind of ought naturalistically? I think that we can, because once again I deny that "wrong" has any prior metaphysical meaning. The trick is to make the new (by definition) meaning not clash too harshly with the existing metaphysical connotations.

How is this for a first attempt at a naturalistic definition of the moral ought as a subset of the practical ought? An agent morally ought not to do something iff it tends to generate consequences contrary to the agent's interests, those negative consequences arising from the reactions of disapproval coming from other agents.

In general, it is not difficult at all to define either kind of ought naturalistically, so long as one is not already metaphysically committed to the notion that the word 'ought' has a prior metaphysical meaning.