Unfortunately, Undead without LA and negative levels on your fists are pretty close to mutually exclusive. Several Undead templates let you deal negative levels with your unarmed strikes, but all have LA. Without that, the best way to get negative levels on your attacks is the Soul Eater PrC (BoVD), but that requires you to be alive. The only remaining option is a ten level investment into Shadow Sun Ninja, which pretty much forces you to go Swordsage (not that it's a bad option) instead of or in addition to Monk. You MUST be Good aligned to enter, but the PrC does advance Monk abilities. The capstone ability lets you deal negative levels on unarmed strikes, but is only useable once per day and for 1 minute at a time.
And a hint: if you use Swordsage, the Unarmed Swordsage makes a better Monk than a Monk does. I'm not kidding.
So in summary, here are your options:
1. Suck up some LA on the Undead template.
2. Don't be undead (or Humanoid), go into Soul Eater.
3. Play a Swordsage and go into Shadow Sun Ninja.