Nice! I figured it out. I was messing around with searching, and this text popped up, after I had changed the number of the search ID (which apparently doesn't work every time, but when the search ID is certain numbers, it works...weird...):
http://forums.macnn.com/search.php?action=simplesearch&showposts=0& ;query=[Search Query]&sortby=lastpost&sortorder=descending& searchdate=-1
It then immediately switched to:
http://forums.macnn.com/search.php?s=&action=showresults&searchid=[Search ID Number]&sortby=lastpost&sortorder=descending
Yay! I have my search link!
I just added it to the QS Wiki. I feel so...accomplished!
Edit: Woot! I went through the HTML of the MacNN News search page, and by looking at the names of the fields around the submit button, came up with a search string.
For the past two weeks of MacNN news:
http://www.macnn.com/search.php?action=post&Submit=Go&checked[1]=&term=[Search Query]&search_limit=1099285200
For no time constraint:
http://www.macnn.com/search.php?action=post&Submit=Go&checked[1]=&term=[Search Query]
Enjoy

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Edit: To search by username (exact name), it's:
http://forums.macnn.com/search.php?action=simplesearch&showposts=0& ;searchuser=[User]&exactname=yes&sortby=lastpost&sortord er=descending&searchdate=-1
For partial name, it's:
http://forums.macnn.com/search.php?action=simplesearch&showposts=0& ;searchuser=[User]&exactname=no&sortby=lastpost&sortorde r=descending&searchdate=-1