I've only written .obj loader for handling 3 or 4-sided polygons, nothing more, but meshes with polygons more than four sides are rare so it has worked for me.

Here's what you have to do to handle 4-sided polygons as triangle lists. In your face-reading routine (that is, where you start adding nodes) count ahead to see if there are exactly 4 integer groups separated by spaces, that's when you know you have a 4 sided poly. Then read ahead and store these values into separate 4-sized arrays and add SIX vertices for the face, as two triangles resulting from splitting up the quad.

Basically what you have now is this:

for each line
- split line by spaces
- if line has 3 number groups
--- for each number group
----- add vertex index
----- add UV index
----- add normal index
--- form a triangle with the three groups of indices
- go to next line

You would want to change your method to something like this:

for each line
- if line has 3 number groups
--- use a polygon index array { 0, 1, 2 }
- if line has 4 number groups
--- use a polygon index array { 0, 2, 3, 1, 2, 3 } // this set of numbers will vary
- for each index in array
--- add vertex index for this number
--- add UV index for this number
--- add normal index for this number
--- if we are in the third or sixth index in the array
----- form a triangle with the three groups of indices
- go to next line

The reason why the set of numbers may vary for 4 sided polygons is because you have to know where the winding order is, and how the triangles should be faced.

If for instance we had a square with vertices 0, 1, 2, 3 starting from the top left and going clockwise, we can divide it in the diagonal from 0 to 2 to get 0, 2, 3, 1, 2, 3. But if we were to divide the quad along the 1-3 diagonal, then our list may be 0, 1, 3, 1, 2, 3. I had to guess which way was the right order for the quads, but you can get it in one or two tries if you know how the triangles should face.

I'm trying to write an .obj loader that will handle any .obj file thrown at it...

I suppose you mean "will handle any polygonal .obj file thrown at it". The stuff revolving around curved surfaces is incredibly modern in my opinion.

I'm beginning to think that .OBJ is BAD.

It sure has several limitations and some quirks. Like most file formats. Managing this is relatively easy compared to other issues going around file formats. It is worth noticing that if polygon triangulation is a problem, it will likely be a problem with other formats as well. In the meanwhile, just hit the "export as triangles" option in the DCC tool and live happily with them.

Consider doing something else and save your time using AssImp.