Unarmed Strikes count as natural Weapons, right?
Books used:
PHB 3.5
Races of Ebberon & Destiny
Complete Adventurer, Scoundrel & Warrior
Tome of Battle
Changeling Rogue 1/ Monk 4(Swashbuckler 3)/Changeling Rogue +1/Warshaper 5/ Chameleon 9(10, if Swashbuckler)
1 Able Learner
3 Ascetic Rogue(Complete Adventurer) or Daring Outlaw(Complete Scoundrel)
6 Superior Unarmed Strike(if Monk) or Imp. Unarmed Strike(if Swashbuckler)
9 free(if Monk) or Superior Unarmed Strike(if Swashbuckler)
12 free(if Monk) or Martial Study: any Shadowhand Strike(if Swashbuckler)[I'd pick Shadow Garotte]
15 free(if Monk) or Shadowblade(if Swashbuckler)
18 free(if Monk) or Martial Stance: Assasin's Stance(if Swashbuckler)
I guess, Swordsage could also work instead of Monk(Unarmed Swordsage Variant may get you both: more Shadowhand AND Imp. Unarmed Strike for free).
Also Warblade instead of Swashbuckler, but then you would lose out on continued Sneak Attack
