See forum discussion
http://moodle.org/mod/forum/discuss.php?d=170353

I propose that:

1) Change to the 'Page contexts' option so that it additionally lets you select 'entire site' and every parent category within the dropdown. At present it only lets you select up to course level.

2) These options only appear if you have the same permission that you would require to edit blocks at category / site level.

I haven't looked at how hard this is to code, probably I don't have time to code it, not sure, but I wanted to record the suggestion in tracker anyhow.

Note this is related to MDL-26105 but that one seems a bit of a nightmare so I thought a more specific/limited tracker issue might be useful.