The README for rgdal contains the following text:
For additional drivers, users of the CRAN Windows and CRAN OSX binaries can either convert to an available GDAL driver externally, read using CRAN rgdal binary - here you factor out mismatches, especially 32/64-bit questions among others, and can check the stages of the workflow. You can script the first step in R through system(). The PATH environment variable will need setting correctly to detect the external software properly.
But I can't find more detail about how to accomplish this, or what is entailed in setting the PATH environment variable "correctly".
I realize that rgdal binaries exist, but I need access to the PostgreSQL/PostGIS driver, so the internal GDAL that is statically linked is insufficient.