I logged NONE of my training time, since I was "appropriately rated" for NEITHER crew position. I could have logged "flight time" but I chose not to do so.
You CAN log sole manipulator time on part 91 legs, but I would do so only AFTER becoming a required crewmember. I'd also clearly put PART 91 in big letters on the comment line when doing so.
And as Falcon Cpt said, only do this when you are typed in the aircraft. When you do your type, ask for a PIC ride for a PIC 8410.