The output of the yellow elec pump is grossly insufficient to run Y plus via the PTU G. Also not enough to run Y with loads. Numbers were mentioned once here on PPRuNe.
At wind milling speeds, true, there will be actually enough pressure on both Y and G from EDPs. Dual ENG does not give you dual HYD with it. Luckily.
So probably, it is the electrical configuration that does not permit AP use, by design. I.e. on purpose and for some very good reason.
My 2c, cheers.