There is no real difference between Cycles and Fragmentarium as far as path tracing is concerned (let’s not go into too much details here), except that Fragmentarium is GPU only (actually GLSL), which should explain the speed difference with OSL.
The approach that Fragmentarium is using to render the fractals is based on distance fields, which tells for each shading point how far the surface is. This is similar to level sets, so when that is supported in Cycles (through OpenVDB e.g.), it will be able to render such fractals, in theory. In practice you would need a way to generate the distance field from Blender.