Good morning, Kori!
Thank you so much for your question – it’s a great one! There isn’t too much written about shaoyin headaches, even the Bensky and Chen&Chen don’t say much about it in their commentaries. My understanding is the same as yours, that it is a headache felt ‘inside’ the whole head, and it could also be a headache involving the teeth. This makes sense when we think of Xixin’s effect on the Kidney channel, and that the Kidney corresponds to bones/teeth and the brain. In terms of accompanying symptoms, I would look for signs of cold and/or blockage since Xixin is a warming herb (found in either the pungent and warm herbs that release the exterior section, or the interior warming section), and it unblocks and disperses.
Let me know if you have any follow up question!
Have a lovely day!