I don't think it has ever been done. 300 lbs is a lot more than most people realize, and I don't think most of you are giving it the respect it deserves.
Here, as a reference, check out some of these powerlifting results from meets here in California:
http://www.powerliftingca.com/meetresults.htm
Some sample results:
Venice Beach Powerlifting, September 2005
148 lb class champ: 391 lb bench (named best lifter)
165 lb class champ: 303 lb bench
181 lb class champ: 325 lb bench
198 lb class champ: 341 lb bench
West Coast Benchpress and Deadlift Championships, June 2005
165 lb champ (Open Men): 220 lb bench
181 lb class champ (Open Men): 303 lb bench
220 lb class 3rd place (Open Men): 336 lb bench
Now, I have no idea what the quality of these meets are. Are these local competitions, regional, state-level? Who knows. But you have to figure guys who compete in stuff like this train for it and are pretty good at it. Plus, all the people listed here are the champions of their weight classes (except for the 220 lb 3rd place guy). So if the 165 lb class champ is only lifting 303...and then at another meet the 181 lb champ is only lifting 303, and the 3rd best in the 220 lb division is only lifting 336...I think that should give you a sense of how difficult benching 300 lbs is. Plus I'm willing to bet that the smaller, weirdly strong guys are really densely built with muscle - haven't we all known people like that? Like those dudes who wrestle in the 150 lb weight class in high school who look like they should weight 190.
Now think about how many people have ever run sub-4. Now think about the upper weight range for being able to run that fast - 175? 180? There have probably only been 10 guys or less who were that heavy who broke 4. Now do you think these same guys could dedicate the time to powerlifting as the dudes in the results listed above? Or have the same body types as the light, but really strong guys.
No on all accounts. Can't be done...actually, no, I think it's possible in theory, but I think those genetic freaks would be so rare that it hasn't been done.