I would never try this on the cittern (because I know it just wouldn't work), but today I tried using a thumbpick on the oud, inspired by the thinking of this thread. I don't use the thumbpick anything like I would on a guitar, of course. I place it on my thumb in the usual way but hold my hand as I would for a standard long oud plectrum. It works beautifully. It is in precisely the same position I would normally have it for an oud, but it gives me more freedom, since the pick is attached to my thumb, there's nothing to grip.

Completely 'inauthentic', of course, but hey, someone at some point must have thought of cutting up plastic bottles. And it works for me.

Thanks, all.