Thank you for at least responding -- apparently none of my other efforts even merited a reply.
Thank you also for pointing out that the Mick Jagger example wasn't representative.
For the other direction in yoga I might actually have tried something like 'arch downward
.' Just with 'arch one's back' alone, I don't think anyone would think of downward, because it normally implies upward.
On a related note, a horse with a downward-arching back is called a swayback
or swaybacked. Siehe auch: Durchhängerücken
That's also missing from LEO, but far be it from me to try to add it if additions aren't actually wanted.