Bypass a linear voltage regulator by applying a voltage at the output while input is not connected?
Although the datasheet doesn't mention it, you will find that most voltage regulators these days, have a protection diode between the output and the input, in order to allow for the capacitors on the regulated side to discharge. This also works if you were to apply an external voltage to the output of the regulator, and therefore it should not damage it.

As pointed out in the comments, there is no certainty about the presence of the internal diode, so to be on the safe side, you should add a Schottky diode between the output and the input.