For lurkers...
Well, after playing with this most of the afternoon I found that if you set customBackgroundColor property, then it will undo any custom background you draw. So, as long as you don't set customBackgroundColor, you can create whatever kind of brush you want (LinearGradientBrush) and just call CellDrawInfo.g.FillRectangle( newbrush, info.cellRect); e.g.
Public Sub OnBeginPaint(ByVal info As CellDrawInfo) Implements IStyledNode.OnBeginPaint
Dim myBrush as Brush
Dim myGradientBrush As LinearGradientBrush = New LinearGradientBrush(info.cellRect, Color.White, Color.Blue, LinearGradientMode.ForwardDiagonal)
' The RotateTransform method rotates the brush by the user specified amount
myGradientBrush.RotateTransform(0)
' Set the point where the blending will focus. Any single between 0 and 1 is allowed. The default is one.
myGradientBrush.SetBlendTriangularShape(1)
myBrush = myGradientBrush
info.g.FillRectangle(myBrush, info.cellRect)