This page says:
The pHAT DAC provides a high-quality digital to analog audio converter for the Raspberry Pi: 24-bits at 192KHz via the I2S interface on the 2x20 pin GPIO header.
According to this description and the image of the pinout as shown there the pHAT-DAC only uses the GPIO pins tied to the I2S interface, specifically the pins 18, 19, and 21 (BCM numbering)... well, that and +5V, but that pin is not used exclusively by the HAT. All other pins are free to use.
I would be reluctant about the use of the two special pins (ID_SC and ID_SD) that are reserved exclusively for attaching an 'ID EEPROM' which contains data that identifies a HAT. However the description of the pHAT-DAC does not say anything about using it. Furthermore the images of the PCB do not show an EEPROM and the DAC itself does not seem to include any neither. From all I know it does not use the ID-EEPROM scheme and thus the pins ID_SC and ID_SD are also safe to use.