Hi Neil,
Unfortunately I don't know much about OpenRoad, but I was responsible for adding some service so that Civil products could use the raster service to handle a wider range of different DEM raster file format that was previously available in older version of Civil.
The functionality was a DEM raster import functionality, reprojection was available, as well as data filtering.
The functionality was added to the Data Acquisition module added if I'm not mistaken, in SELECTSeries 1. I think the Data Acquisition is now called Survey in OpenRoad.
Even if you have a 12 GB machine OpenRoad cannot use more than 4 GB because it is a 32 bits application. By experience the new terrain model element can stored about 5-10 millions points maximum.
If you don`t want to filter out the points the solution would be to create smaller DEM grid and work with one grid at a time. If you have access to the latest version of Bentley Descartes (SELECTSeries4) you could create a Scalable Terrain Model element in the desired coordinate system from the DEM raster files you want to use and extract a multiple of TM from that STM.
Otherwise with older version you could try the corridor tool to create smaller DEM raster.
Thanks,
Mathieu