Thanks for the feedback LoyceV, less than 10W is absolutely impressive and far lower that I'd have expected.
Shouldn't ARM-based laptops (Chromebooks, Windows 10 ARM, ...) even be able to undercut this value? Whereby this is of course a question of the software, especially for this purpose the software will probably not be executable for ARM.

In any case, it's an interesting thought experiment and is now making me reconsider my Raspberry purchase. The Thinkpads in particular should be quite suitable for a low-watt strategy due to their processors ... I will definitely have a closer look into this topic now.