As the model A supports back-powering through the USB port, you will bypass the fuses. So it is not a good idea in general.
BUT. As long as you do not connect other devices to GPIO and as your only USB port is occupied, you will hardly run into the situation that you route too much power through your Pi. So to my best knowledge your Pi will be save but potentially underpowered. Smartphones don't provide much power through USB OTG. My iPhone is limited to USB devices about 100mA (showing an alert stating a device draws to much power otherwise) which is WAAAY to low for a Pi. As the USB port on any smartphone is protected you will also be save on that side.
As I am confident that it will cause no damage to any of your devices I don't take responsibility. To my best knowledte your Pi will be underpowered and run unstable (potentially damaging the file system on the SD card).