WindowsFormsHost ZOrder

Unfortunately no, because of the way the winformshost is composited into a WPF window it must appear on top.

See the z-order paragraph from here.

In a WPF user interface, you can change the z-order of elements to control overlapping behavior. A hosted Windows Forms control is drawn in a separate HWND, so it is always drawn on top of WPF elements.

A hosted Windows Forms control is also drawn on top of any Adorner elements.