1、LTC6803 In most applications, are the batteries under test used for direct power supply？
2、Can VREG use as a power supply for a communication isolation chip in use of SPI？
1) V+ (Supply) pin can be tied to the most positive potential in the battery stack.For further info please see pg 9 and application examples of the datasheet.
P.S. another way is to use isolated power supply depending on your application requirements. There are also examples of that in the datasheet.
2) VREG is for some small external loads.Typically up to 4mA, please see pg 8 and 10 of the datasheet. (Please note that VREG can only source current, NOT sink). You can look at different isolators from Analog Devices. For example micropower family (ADUM144x).