CDCE706: ISP possible or not?

Yes, the device registers and EEPROM can be programmed in-system via the SMBus data interface.  The SMBus interface, data protocol, and registers are described in the datasheet.  The device register settings can be generated using TI's Pro Clock software GUI and exported a hex file, so it can be programmed by your system host. Please see this E2E post for links to the related software: 

https://e2e.ti.com/support/clock-and-timing/f/48/t/773187

Alan