I've read that both Craven and Staggering Strike are very good feats for a Sneak Attack oriented swordsage, however, per RAW, I don't think knowing Assassin's Stance would accomplish the prerrequisite of being able to make sneak attacks, since it's more of an "added" ability rather than a "natural" ability like, for example, the Rogue has. So, as the title says, would RAW allow that? It could easily be houseruled, however my DM is already giving me the stink eye by multiclassing into Swordsage.
Assassin's Stance gives you Sneak Attack, but not as a class feature.
The Assassin's Stance maneuver (ToB, p. 75) says (emphasis mine):
While you are in this stance, you gain the sneak attack ability, if you do not already have it, which deals an extra 2d6 points of damage. If you already have the sneak attack class feature, your existing sneak attack ability deals an extra 2d6 points of damage. See the rogue class feature for a complete description of sneak attack.
It doesn't get much clearer than that - Assassin's Stance gives your character sneak attack. For some feats, this is all you need. For instance, Staggering Strike (CAdv, p. 112) has the following prerequisites:
Base attack bonus +6, sneak attack
You clearly meet the sneak attack prerequisite while Assassin's Stance is up, and can take the Staggering Strike feat accordingly.
If you're worried about the fact that you only have sneak attack while the stance is active, take a look at these two questions about meeting prerequisites using bonuses that may not always apply. The rules don't address this quite as clearly as they might, but player consensus and FAQ rulings suggest that at worst, you might lose access to a feat you qualified for this way while not in Assassin's Stance. You would still be able to qualify in the first place to select the feat, and could regain access to the feat simply by re-activating Assassin's Stance.
However, not all feats have such friendly prerequisites. For instance, the Craven feat (CoR, p. 17) has the following prerequisites (emphasis mine):
cannot be immune to fear, sneak attack class feature
A character with Assassin's Stance has the sneak attack ability, as described above, but they do not have the sneak attack class feature. A class feature is something you gain from the benefits described in your class, not something you gain by using an ability like a martial stance. Thus, by a strict reading of the rules, a character with Assassin's Stance but no other source of sneak attack would not qualify for the Craven feat.
As a DM, I personally would let this slide. I find the hair-splitting between "the sneak attack ability" and "the sneak attack class feature" to be unnecessarily pedantic, and pragmatically I haven't found sneak attack to be strong enough for balance concerns. However, this would be a house rule on my part.