Hi Stewart,
What are the lenes and filter sizes of the lenses that you are using/will use with the camera?
If the filter size does not exceed 82mm and the camera has sufficient clearance, you may want to consider the Cokin P size holder. Lee and Singh-Ray make extremely high quality resin filters (including Grad ND) in the 84mm X 120mm (Cokin P) size for this filter system. Additionally, Singh-Ray makes a glass Warm Polarizing filter to fit the rear of the Cokin P Holder that allows it to rotate in the rear slot of the holder. Both Lee and Singh-Ray also offer many other filters both 84mm X 84mm and graduated (and non graduated) 84mm X 120mm for the Cokin P holder.
Another possibility is to contact Lee and ask if they could make the grad filters for you in the 100mm X 100mm size, but that will limit your ability to adjust the position of the gradation.
Good luck.
Rich