We show that changes in transmittance, in TFBG hydrogen sensors coated with nanometric metal/metal-hydride coatings with highly lossy cladding modes, are primarily a function of the mode losses, which we demonstrate, can be well estimated from the coating reflectivity. Using this result, and a simplified set of coupled mode equations that depend only on the mode losses, the sensor transmittance is recovered. We use this computationally fast model to show how the change in resonance amplitudes with hydrogen loading, and the envelope of resonant peaks, is tuned via an multi-layer metal/metal-hydride coating.