Thanks for the patch. Setting the parameter works now, although not for all values.
I wanted to be sure I have not done anything wrong when modifying the config.c file, so I also tried dowloading the latest version of libgphoto2 from the SVN trunk. Setting the parameter fails for the following values: 2, 6, 10, 14, 18, 22. The debug output says:
0.594405 ptp2/usb_getresp(2): request code 0x1016 getting resp error 0x200f
0.594439 context(0): The property 'ISO Auto High Limit' / 0xd183 was not set, PTP errorcode 0x200f.
*** Error ***
The property 'ISO Auto High Limit' / 0xd183 was not set, PTP errorcode 0x200f.
0.595598 gp-camera(2): Freeing camera...
0.595680 gphoto2-camera(2): Exiting camera ('Nikon DSC D3s (PTP mode)')...
Here's the complete output of 'gphoto2 --set-config /main/other/d183=10 --debug' (svn version):
Perhaps there's a logical explanation why every fourth value fails. Anyway, I can already start working with what I have now.