How many levels do you have to play with? This is important because there is a prestige class called the Shou Disciple which gets at 5th level of the PrC Martial flurry - any. Allowing you to flurry with any martial weapon. Since katana is a bastard sword you could use it with such. Plus if you play with regional feats you would be able to start with a MW Katana too. Catch is you need a +5 BAB so you'd need at least 8 levels to even come close.