Can you tear open and use the contents of a tea bag as makeshift loose-leaf?

You could, but it isn't advisable as the "tea" in a tea bag is closer to powder than to loose leaf tea. When making tea bags the companies tend to use the parts left at the bottom of a loose leave style of tea. I have yet to find a tea strainer that will take the "bits" out of the tea.

Even if your purpose in using the tea is to "read" the leaves, the powder is so fine the reading is rarely accurate.