Low-power interrupt generator
Not only is this scheme supply voltage dependent as you've noted, but it also depends on the precise atomic-level geometry of the input transistors, specifically their leakage and voltage threshold.

If you want even lower power than the WDT then you should consider using a watch crystal and the asynchronous timer. The datasheet puts typical power consumption for this method at 0.8µA at 1.8V and 0.9µA at 3V, making an extrapolation of 1.1µA at 5V very reasonable.