I dont think this is possible. When the ECO is enabled, the oscillator circuit gets connected to P1 and P1. So, even if you are not using the XTALOUT pin to drive a crystal, this pin can not be controlled independently through PRT1DR register or by the Global bus as there will be a contention with the ECO circuit.
This is not possible because whenever you connect a crystal and enable the same, then the internal circuit which is generally a inverter complements the line and output it to XTALOUT line. Thus, even though you are not connecting using the XTALOUT for ECO purpose but you won't be able to use for any GPIO purpose becaus einternal circuitry drives this line.