@dtex I think the short answer is "probably not a good idea" because the T2 is only rated to 5.5V. It may not cause immediate damage to the T2 but it could degrade performance over time.
What's the power rating on your battery and how many servos are you trying to power and what kind of load do they have? @ekolker shared some power characteristics of the T2 here. Each of the servos and the Tessel 2 will need some supply of current to keep them running. If the battery isn't able to supply the current needs of each element, the voltage will start dropping below 6V. If the voltage gets too low, the Tessel will shut off.