Could HDMI be potentially used as a power supply alternative for smartphones?
The specification of the +5V charge is 50mA(according to wikipedia) This is even lower as USB before enumeration(100mA) Any device wouldn't be able to charge , or it will be very slow at 50mA (10x slower than normal USB port at full power, 500mA)

I don't think the HDMI connectors are specified for high currents in excess of 0.5A. Furthermore, the +5V is probably used in the monior to power some small circuits for identification of itself to the PC/television/etc. , even if it's turned off.