I am guessing that the reason for not allowing the personalized green plate is to have a redundancy - both the green colour of the lettering and the number starting with GV or VE. Sometimes it is hard to tell the colour, especially if you have plate covers on. So there is some logic to it.I got a personalized plate (blue) because green plates do not have the option to be personalized. I drive about 3-5 times a week on the 404 HOV lanes. So far I have not gotten stopped by a cop, though I have not seen many cops monitoring the HOV lane during the times I was using it.
I find it unfair for there not to be a personalize option for green plates and if I ever get stopped I'll bring that up with the officer.