OK, thanks silver18 for your other post about the orientation, which inspired this patch.
Here is a patch which allows to use *all* orientations (including upside-down) in the reader, the home screen, and even Apps (Kindlets). It does not require any changes to the system configuration. (except the jbpatch; download in first post of this thread).
Note: it does not affect WAF apps such as the browser.
Source code is here
This file is already included in newer versions, so there is no need to download it.