slowrotor,
IMHO, there should not be any significant difference between a roll maneuver and a pitch maneuver. The fuselage moment and the HS will offer a greater resistance to pitch than to roll and this might result in the 2/rev pulse during pitch being a little less noticeable.
I have not found any reason why a two-blade rigid hub with flexible blades would not work on a full size(ultralight).
The hub spring will act similar to a rigid hub with flexible blades. This page give information related to the hub spring and attempts by others, such as Bell, to overcome the 2/rev vibration.
http://www.unicopter.com/1230.html
Dave