QGIS » QGIS Application

How GRASS is involved here? There is rgdal. Especially if you start with r.external, I don't see any reason it should be somehow linked to GRASS plugin. It can be general tool either reading data directly from providers (better I think) or via gdal (rgdal).

Sorry I was unclear. Many users find the location-mapset GRASS concept difficult to understand, and cumbersome for one-shot analyses.
The idea here is to hide this complexity by creating a location on the fly, running the analysis, exporting the result, and load it on the canvas.
In this way, one could use GRASS without knowing much about it.

See more details in #3135

Concerning on the fly GRASS locations, see this example of a simple implementation: http://grass.osgeo.org/wiki/GRASS_and_Sextante

Sextance uses GRASS in a very efficient way, no need to understand GRASS' location-mapset concept. It creates a mapset on the fly and just runs the analysis in a temporary session as indicated above. Works nicely in the gvSIG-OADE-Sextante-GRASS package which I indicated in the GRASS Wiki page. It would be a BIG (and likely rather easy to implement) improvement of the QGIS-GRASS integration to use the same method. All pieces are likely there.