Is there a method to do ray marching style modulus repeat with raytracing?
I've tried to figure this out as well, but I don't think there is a solution that is just as performant as tracing one sphere.  Basically you would want to cast one ray to hit a infinite set of repeating spheres with no marching iteration.  That would be awesome but is not possible:  Let me explain.

 

Ray-tracing is basically finding the analytical solution to this equation:
$$
f(\vec{r(t)}) = 0 \ for \ t
$$

Where $f(\vec{p})$ is the implicit surface function (which gives the distance the given point $\vec{p}$ is from the surface) and $\vec{r(t)}$ is the parametric ray equation ($\vec{r(t)} = \vec{origin} + \vec{direction}*t$).  $t$ would represent the distance along the ray the intersection happens.

In order to ray-trace domain manipulations (like repeating space), these domain manipulations need to be present in the implicit surface function $f(\vec{p})$ while finding an analytical solution.  Let's take a 3D repeating sphere for instance.
$$
f'(\vec{p}) = \|\vec{p}\bmod \vec{d}\| - r
$$
(where $\vec{d}$ is a constant representing the grid size and $r$ is the radius)

In order to solve $f'(\vec{r(t)}) = 0$ for $t$ we would need the inverse of the modulo operator which doesn't really exist : http://mathforum.org/library/drmath/view/51619.html.  That means there is no analytical solution to this equation, but there is a numerical one. Hence Ray-marching.

 

This is true for a lot of cool distance functions you see on shadertoy.com, the analytical solution does not exist for these surfaces so you cannot ray-trace.