Everything that follows is my opinion, based on my experience with Hero from 3rd Edition on. Don't go building official characters with anything posted here....
Typically, the advantage of Based on ECV comes in from three sources. First, most people have no Mental Defense, second, EGO (the targeting characteristic) costs less than DEX, and third, most people have lower EGO than DEX, resulting in a lower DECV. The description of the psychokinesis power in the FREd (p147) says that the target still uses DEX based DCV to avoid being hit, thus eliminating source number 3.
Common sense implies that being hit by a thrown object (no matter whether that object is thrown by hand, by magnetic beams (a SFX of standard TK), or by the power of the mind (psychokinesis)) will hurt a brick less than a physically frail mentalist, ie PD applies, not Mental Defense. Advantage source number 1 is out.
Based on the fact that the only real advantage involved in this case is that fact that the character targets the TK with the relatively cheaper EGO rather than DEX, I'd only charge the character a +1/2 multiplier instead of the standard +1.
Like I said, I doubt this would float in any sort of official setting, but in my personal games, I'm not out to screw my players.