I wouldn't do that with all weapons. Swords like you might by at the mall for instance certainly don't qualify. But a club is a very simple sort of weapon and baseball bats are just very well balanced clubs, almost the epitome of clubishness.
I know, mall swords are crap (I study metallurgy, one time some guy on the forums tried to tell me you don't temper metal for swords, and that harder metals weren't sharper. I could have made a a whole page just filled with a rant, but I felt it wouldn't be wise to argue with somebody who clearly doesn't know about the subject matter, but thinks he does.), but when I said modern weapons, I didn't mean modern methods of making older weapons like swords, I meant modern weapons/tools of the modern time like bats, cricket bats, golf clubs, machete (which I would use as just a shortsword, or some sort of light weapons), sledgehammer, so on so forth. I could see how somebody would make a sledgehammer a greatclub (wouldn't make it that myself, I would probably make it a custom weapon with the same stats but possibly a different crit chance and the blocking special, and maybe trip), but I wouldn't consider it a masterwork greatclub. Maybe if it had an African ebony handle with a head that had some sort of scifi vibro absorber in the head, then maybe. If I considered a baseball bat a great club (which I don't, I think Tovec had the best way of handling it personally), I wouldn't make it masterwork, I don't see how a aluminum club counts as masterwork, and if its wood, its the same as another greatclub. I could see it as being masterwork if it were the http://www.coldsteel.com/Product/92BS/BROOKLYN_SMASHER.aspx , than yeah, that's a masterwork product, but your common baseball bat is not at all masterwork in my personal opinion .
I agree that a teeball bat could work as a club (but I think light mace works better, but idk), but I don't see any reason for it being masterwork, for the reason I follow, #1: I could get one at Target for $15.09, and it would be a aluminum, hollow tee ball bat that wouldn't be any better at causing damage than any other club, but if it were made from heavy-grade polypropylene (I'm a fan of Cold steel, sorry), ebony, or walnut, then yeah, its harder/or superior to the lesser product. One homebrew I've come up with is a special material Titanium: Titanium in metal weapons makes it a +5 masterwork item, but has the fragile condition when up against heavily armored foes.

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