It looks like the ability to create rasters is not yet available in the driver. However, you can create a layer using your standard PostgreSQL Python driver (i.e. psycopg2) and then open it from GDAL.
So you'd create a layer using ST_MakeEmptyRaster and ST_AddBand:
CREATE TABLE rtest (gid serial primary key, rast raster);
INSERT INTO rtest (rast) VALUES (ST_AddBand(ST_MakeEmptyRaster(100, 100, 1, 1, 2), '8BUI'::text));
Then you'd open and manipulate the raster in GDAL/Python
ds = gdal.Open('PG:host=localhost dbname=mydb table=rtest user=myuser')
I haven't tested this extensively yet, but it
seems to work. doesn't work at all. I get the error,
Writing through PostGIS Raster band not supported yet
when I try to write.
You could use Python and psycopg2 to write chunks of your image to a raster table using ST_SetValues.