you would name the joint mountPoint, and you would mount the .dts to "$HeadSlot"
This is from my Gas Mask:
datablock ShapeBaseImageData(GasmaskMaskImage)
{
shapeFile = "./Gasmask.dts";
emap = true;
mountPoint = $HeadSlot;
offset = "0 0 0"; //Change this to position the model on the head, viewed in third person.
eyeOffset = "0 0 -0.05"; //Change this to position the model in first person.
rotation = eulerToMatrix("0 0 0");
eyeRotation = eulerToMatrix("0 0 0");
scale = "1 1 1";
correctMuzzleVector = true;
doColorShift = false;
colorShiftColor = "1 1 1 1";
hasLight = false;
doRetraction = false;
};