If a non verizon person calls you and you forward it to a non verizon number I believe it counts as 2MOU per min.
Bottom line though is this, if someone calls you and hangs up when they hear your voicemail that is 1 min... so I believe that if a non VZW calls you and hangs up even if you have your GV number saved you will still get 1MOU billed.
Also I wouldn't be suprised if VZW reserved the rights to be able to not give you the FnF on the GV access number... not your GV number for voicemail but the one that is called when you dial out using you GV number. I do know that when I used YouMail it seemed that even with the youmail number in my FnF I was billed a lot more minutes than usual. I now use GV and do not seem to notice that for the voicemail as much, but who knows...
Use this carefully and make sure that if you go over your minutes you point one huge finger at yourself and not VZW.