Isotope-selective photodissociation is a promising route to laser-based separation, yet its efficiency remains constrained by fixed molecular cross sections. Here, we introduce an active optical-switching strategy that utilizes a nonresonant ultraviolet control pulse to generate and tailor isotopologue-specific Fano resonances. By coupling high-lying vibrational levels of the electronic ground state to a dissociative continuum, this pulse dynamically modulates photodissociation cross sections without direct electronic excitation. Using full quantum wave packet simulations of HF and DF isotopologues, we demonstrate that the photofragment yield ratio can be reversibly switched by orders of magnitude through tuning of the probe frequency across a light-induced resonance. This approach enables selective suppression or enhancement of dissociation for a target isotopologue with high spectral precision. Our work establishes a versatile and efficient mechanism for isotope-selective photochemistry and opens a pathway toward coherent optical control of molecular photodissociation.