Albeit you apparently can't add the Backup Volume or TimeCapsule's SparseBundle Volume to the Privacy List of Spotlight via the '+' command, to me it worked very well to just drag the volume's icon to the Privacy window of Spotlight where it appears if not directly then at least the next time I open that very window.

Works like a charm for me with 10.5.5 and bot a PowerBook G4 and a MacBook Pro 15".

Best regards,
Michael

Thanks for all the hints and comments.
Of those who got it back to work or who can add the backup volume to the privacy list, is anyone using this particular Backup Disk: "WD Mybook World Edition" with Firmware 02.00.18?
I had been trying for the longest time to fiddle with the .Spotlight-V100 directory of that drive, deleting it to have spotlight rebuild it, using the mdutil framework etc. It showed, in general, very odd behaviour in the sense that indexing could not be enabled or disabled.
I also cannot drack and drop it. It gives me an "unknown error" when I try.
@drmacnut: I can browse through my older backups without much problem. It takes some time, for particular dates to refresh in the finder window, but this is not surprising, as I have my Mybook now access the backup over the WLAN.
I will dry to check the disk image using the disk manager and by hooking up the computer to the ethernet, again. If there is something corrupted, I will post it here.