Hey Dennee,
Am in total agreement with you that Newari metalwork should be included because of their close relationship with Tibet. Alos believe Indian metalwork very closely related and there are many accounts of Indian arms and armor being used and imported into Tibet. In fact from 700-900 AD that area was part of the Tibetan Empire and Tibet was larger than China at that time. However my focus is from 1300-1900 during which the borders of Tibet remain fairly constant.
The Metalwork is virtually the same from Bhutan and Nepal at times, with differences apprearing in style and not decoration. In arms and armor Tibetan, Nepalese and Bhutanese items are very scarce prior to 1600, but that the same time they seem to be at the height of their art then too.
The advantage of using Chinese Art as a reference is that besides being a huge influence on its neighbors they is also quite a bit of study done in ceramics and textiles that you can use the trend in design and petterns used to help date Tibetan metalwork by comparing designs and art work.
Link below shows old maps and time periods of Tibet...
http://www.tibetmap.org/pw4br.html
rand