Is it feasible? How much change in continuum flux is needed to see a change in photometric magnitude?
Here, is a random quasar spectrum, taken only in the g-filter passband (3800-5500). What I did was take the region between the CIV and SiIV emission features and played with how much drop in flux is needed for there to be a significant change in the measured g magnitude.
The only difference between the red and black spectra are that (in this region) the black spectrum is 50% lower. The resulting change in g_mag is about 0.09. This is just above the limit of detecting changes in the magnitude. Given that the error on SDSS measured magnitudes is 0.02, we should be able to see changes at the 0.085 mark. Significant changes may be needed to produce any large magnitude change.