If a ToB Maneuver such as Daunting Strike calls for a melee Attack as part of the maneuver, can this melee attack be resolved as a trip attack/include a trip attempt?
A special attack that can be substituted for a melee attack can't usually be used with a martial maneuver that mandates a melee attack
Tome of Battle on (Type) on Strike says that
In addition, you cannot combine special attacks such as sunder or bull rush with strikes, even if you have feats that make such special attacks more potent. However, some strikes enable you to make special attacks as part of their initiation.... (43)
It's buried in the middle of a technical paragraph, but there it is. That means, for example, an adept who initiates the martial maneuver daunting strike can't replace the required melee attack with a trip attempt.