  We consider a disformal coupling between Standard Model matter and a cubic
Galileon scalar sector, assumed to be a relict of some other physics that
solves the cosmological constant problem rather than a solution in its own
right. This allows the energy density carried by the Galileon scalar to be
sufficiently small that it evades stringent constraints from the integrated
Sachs-Wolfe effect, which otherwise rules out the cubic Galileon theory.
Although the model with disformal coupling does not exhibit screening, we show
there is a `screening-like' phenomenon in which the energy density carried by
the Galileon scalar is suppressed during matter domination when the quadratic
and cubic Galileon operators are both relevant and the quadratic sector has a
stable kinetic term. We obtain the explicit 3+1 form of Maxwell's equations in
the presence of the disformal coupling, and the wave equations that govern
electromagnetic waves. The disformal coupling is known to generate a small mass
that modifies their velocity of propagation. We use the WKB approximation to
study electromagnetic waves in this theory and show that, despite remarkable
recent constraints from the LIGO/Virgo observatories that restrict the
difference in propagation velocity between electromagnetic and gravitational
radiation to roughly 1 part in $10^{15}$, the disformal coupling is too weak to
be constrained by events such as GW170817 or by the dispersion of
electromagnetic radiation at different wavelengths.
