Exploit the fact that the vertices of the dual to a Platonic solid correspond to the centers of the faces of the solid itself. For instance, the dual to a cube is a regular octahedron, and the six vertices of this octahedron are in the direction of the centers of the faces of its dual cube. 

Find the normalized directions of the face centers of the cube (for instance) this way:

 FaceCenters = Normalize /@ PolyhedronData[PolyhedronData["Cube", "Dual"], "Faces"][[1]];

The direction in Euclidean coordinates as a function of spherical angles \[Theta] and \[Phi] are of course:

 x[\[Theta]_: Real, \[Phi]_: Real] := {Sin[\[Theta]] Cos[\[Phi]], Sin[\[Theta]] Sin[\[Phi]], Cos[\[Theta]]}

Now sweep through all spherical angles, and for each corresponding direction find the nearest face center direction (i.e., the one with the smallest angle to the direction defined by \[Theta] and \[Phi]). For each such direction, the distance to the surface is 1/Sin[\[Psi]], where \[Psi] is the scalar angle between the candidate direction and its nearest face direction:

 SphericalPlot3D[1/Sin[x[\[Theta], \[Phi]].Nearest[FaceCenters, x[\[Theta], \[Phi]]][[1]]], {\[Theta], 0, \[Pi]}, {\[Phi], 0, 2 \[Pi]}, PlotPoints -> 100] // Quiet

![Cube fig][1]

If you repeat with a Tetrahedron, for instance, you get this:

![Tetrahedron fig][2]

If you repeat for an Octahedron, you get this:

![Octahedron fig][3]


 [1]: https://i.sstatic.net/FJTih.png
 [2]: https://i.sstatic.net/Ek8gx.png
 [3]: https://i.sstatic.net/sufce.png