There must be data available. At Tyrolean Airways, the flight planning programme is able (and has been for a rather long time) to calculate fuel requirements vs. TAS and enroute time based on at least 4 different power settings. Also, at least empirical data must be there with regards to the use of reduced takeoff power and its effect on engine lifetime.
The "raw" data beyond the graphs available in the OM-B is not made available to crews, only via the flight planning systems. Maybe contacting VO flight operations would be an option?
Empirically, though, I do share Your point of view with regards to reduced power in cruise. Using intermediate or long range cruise power will reduce the IAS by around 20-30kts (TAS effect is obviously FL and temperature dependent) but reduce fuel flow by more than 20%. Of course, headwind will reduce the savings to a point when reducing power may even result in higher total consumption.