Here is a mantra volume shader that procedurally creates volumetric lines.
There are 3 basic ways to create lines in a volume. The most natural one for Houdini artists would be to sample line primitives in a sop context or the shader itself.
Drawing lines directly, with a line drawing algorithm is possible too. In fact it is very efficient if you want to do it in Python, since all you care about is calculating the list of voxels to be shaded. However, I found it hard to find an antialiased version of line algorithms that works in 3 dimensions.
The third method, which I employed here calculates random line positions and stores them as a lightweight texture (just storing vector positions). The texture can then be looked up in a shader. A benefit of this is nearly automatic antialiasing, and line thickness variation. It is also possible to distort the lookup space and thus create wriggly lines, although with big distortions they are very noisy. (I think this has something to do with the sampling space not being distorted in the same way, so you may have to oversample -a lot).
I don't have enough rendering resources (laptop) so some of the images are stills.