is there a way to get QGIS and its projection engine OGR2OGR (GDAL software) to use a distortion grid by default. We are continually using a combination of NZMG and NZTM (NZGD2000 datum) datasets that we want to display and edit. We have noticed QGIS (ver 2.14) shows the same point on both projections to be misaligned by a few metres. It appears ogr2ogr is using the 7 parameter algorithm as the default. We want it to use the nzgd2kgrid0005.gsb distortion grid (which it has in its ..\share\proj folder) by default. I didn't want to have to set up a Custom CRS if I could help it.
Apart of the North American Datum of 1927, no datum shift grid is used in the PROJ/GDAL world by default EPSG codes.
The projection methods for NAD27 and NZGD49 are hardcoded in pj_datums.c. So you have to change it there, and compile PROJ.4, GDAL and QGIS depending on that.
You will find some hints at http://osgeo-org.1560.x6.nabble.com/gdal-dev-How-to-set-GDAL-cs2cs-proj-to-use-NTv2-grid-shift-file-when-using-EPSG-codes-td5053819.html and How to set GDAL/cs2cs/proj to use NTv2 grid shift file when using EPSG codes?.
I guess creating a custom CRS is far better for your work case, even if you want to avoid it.