Upvote:5
If you're neither a student nor employed, don't claim that you are. The visa authorities routinely take lying much more seriously than the thing you're lying about.
Just tell the truth, and make sure to document in your materials that you have a legit post-doc offer in the US that you're going to start on such-a-such date. This ought to be sufficient for the ECO to conclude that it wouldn't be attractive for you to immigrate illegally and settle as an undocumented menial worker.
However, if your host institution is covering not only your travel expenses, but also lodging and food, then I think there's a risk the immigration authorities will consider what you do to be working, which is not allowed on a visitor visa. If you haven't already, I'd strongly recommend getting in contact with the host institution's international office to make sure you're applying for the right kind of visa for your circumstances.