  Resonant elastic x-ray scattering (REXS) is an exquisite element-sensitive
tool for the study of subtle charge, orbital, and spin superlattice orders
driven by the valence electrons, which therefore escape detection in
conventional x-ray diffraction (XRD). Although the power of REXS has been
demonstrated by numerous studies of complex oxides performed in the soft x-ray
regime, the cross section and photon wavelength of the material-specific
elemental absorption edges ultimately set the limit to the smallest
superlattice amplitude and periodicity one can probe. Here we show -- with
simulations and REXS on Mn-substituted Sr$_3$Ru$_2$O$_7$ -- that these
limitations can be overcome by performing resonant scattering experiments at
the absorption edge of a suitably-chosen, dilute impurity. This establishes
that -- in analogy with impurity-based methods used in electron-spin-resonance,
nuclear-magnetic resonance, and M\"ossbauer spectroscopy -- randomly
distributed impurities can serve as a non-invasive, but now momentum-dependent
probe, greatly extending the applicability of resonant x-ray scattering
techniques.
