Welcome to Yorkie Talk. I do not have personal experience with this and only know what I have read. I'm sure people with experience will respond.
Glad you did the pre-op bloodwork and caught the problem. Also glad that you are doing additional testing.
I know from member LadyJane's experience that a dog can go years without exhibiting any outward signs of a liver shunt. The high BATs could also indicate a different liver problem, like MVD which is now called PVH. Here is a thread:
http://www.yorkietalk.com/forums/yor...tobias-ut.html
Sending positive thoughts for good news from the ultrasound. Keep us updated. I should add, these are all problems that have a very high treatment success rate.