In the advance search, it looks like you might be able to stick *.com (where * is a wildcard maybe) in the domain text box. If this is the case, then it allows to you to a partial search on domains that will only find .coms. I did a search on *.com in the domain part, and fishforums.net as the search. This looked like it returned .com's that mentioned fishforums.net, but not fishforums.net itself.
Although this is not fully answering the question as people in the UK and other places register .com's, at least it may narrow down the search. Guess it works better for countires with more specific domains such as .co.uk. Is there a us specific one, because .com is adopted by most.? i'm being thick..
Squid