The ninja class has a little noticed high level ability to poison a weapon as a move action instead of a standard. Could be some synergy with an Xbow wielder, who may have a spare move action each round. Although I'm not sure its worth that many ninja levels...
I'd say to go with something that gives you bonus damage on each shot (bard song, ranger favored enemy, cleric's divine favor, rogue's sneak attack, etc) and do a standard Rapid Shot build. It'll cost you an extra feat (Rapid Reload) but works well with small, low STR races when a bow might not. A halfling, gnome, kobold, or goblin could be effective which such a build.