As far as I noticed, can't confirm it, because it would be too much work, if you use only that person, you want drop for to kill an enemy, the chances that person will get a weapon is much higher, although I must say as you need it for Yuna, using summons screws your chances, also overkilling with person in question ups chances too, at least in my experience, then again, as you need it for Yuna, that's not an option, but using only Yuna and summoning, should help some.
Out of the curiosity, how much you pay Yojimbo to Zanmato it? I rarely get Zanmato when I want it, inspite of sum paid, like 1 in 6-7 times, sometimes 2 in a row, but that's rare occurrence, once he used Zanmato on his own, though, quite unexpectedly and when not needed as I was trying to get his overdrive full, that didn't work quite as planned, lol.