Is it possible in the MAX17504 to have high side switching transistor in an on state, effectively turning off the buck function of the device and feeding the supply voltage to the buck straight through to its output stage?

This would require a 100% duty cycle possibility. The MAX17504 can not be operated at a 100% duty cycle. Thus the gate must be driven higher than that input supply rail in order to turn the FET on when Vout gets close to Vin.  This is done by taking switching energy from the switching node (LX) via the boost cap connected between LX and BST.  This is part of a capacitive charge pump circuit that is used to create a voltage higher than the input voltage. For this to work properly, there must be switching action at the LX note which means that the upper FET has to be turned off and the lower FET turned on periodically.  i.e. the Buck has to keep switching.