How about adding that link to the forum pages too?
BTW, why is it "Search in (all) forums" within individual forums? I would have expected a "Search in _this_ forum"!
I agree that it would be good to have a link to the advanced search within individual forums, also a 'Search in this forum' option. For a long time now there has been talk of moving to ForumNG, and whilst this possibility remains, nobody is inclined to make improvements to the standard forum module.
However, as a temporary measure, many forums now include a link to the advanced search on the new discussion topic page.