I create an object, and then create a sub-object of that object.
For the sub-object, I then load light\point.obj and position it in the world.

However, it's position in the world does not appear to match where I actually place it. For example:

o.finalMatrix().pos and o.matrix.pos both return (-1.21, 0.73, 10.1). However, this location is a long way away from any of the objects in the world, and the correct location is nearer (25.5, 0.5, -85.8).

Not quite sure why. Possibly a bug?

sub objects are not meant for drawing (they don't take parent matrix into account)
please think of them like character items

I understand. But I created a new object type that draws its sub-objects.

Is there no way to determine the location of the sub-object?

You can try transforming sub object matrix by parent object matrix
I think sdk tutorials game folder has shown this with lights tutorial. The one with table, candle and a couple of loading lights in the scene

Okay, I shall give it a go. Thanks