If they had a non-GFCI receptacle outside, would you extend the circuit and add another non-GFCI? The work that you do needs to meet the current code. The code requires a dedicated circuit to the bathroom, so look for your circuit elsewhere.
This is a little different : Ex. say the LR ckt also feeds bathroom. Is it against code to add a receptacle in the LR? Thats basically what the op is asking.
I cannot find any prohibition for that.
The code only says a new bath circuit cannot feed other outlets, it is silent afaict on an existing situation such as the ops.