I want to use Helm of the mammoth lord on a character that makes extensive use of fox shape: mostly for the gore attack. Does this work?
Option 1: Shapeshift with the helm on.
This will work because: The tusks, like the other abilities, are a permanent magical effect Permanant magical effects. You turn into a whatever with tusks.
It won't work because: The tusks rely on the gross physical features of the helm and the helm is subsumed into the fox form.
Shapeshift and then put the helmet on.
This will work because...
A shapeshifted character is not an animal companion or familiar. They do not use the same rules for item slots.
Shapeshifted characters can wear armor, including a helmet. The fox can be put in a full suit of plate armor (though it would only give a +4 armor bonus because he's tiny), including the helmet.
This won't work because.... according to the chart claws/paws critters don't have the anatomy to wear a helmet.