So far I've used one of two methods to georeference public domain maps from nla.gov.au.
The first is to (with the aid of the scripts in here) simply create a list of ground control points using another map with known coordinates. The output of this process is a .points file.
The second is to use gridlines present on the map to georeference to that. This process was again aided with these scripts. The output of the process is a .svg file. A .points file can be generated from these .svg using the scripts in georeferencing-scripts, but for convenience I've also uploaded the .points file for these .svgs.
The grid-distance.csv file should have an entry for each svg file which contains the distance between each gridline in degree (this is actually the horizontal distance, but I've never seen non-square gridlines).
If you already have tools to integrate this georeferencing data into your existing systems, use that. I personally have been using my scripts to warp these to subsequently push them out to a mercator tile directory or to a GroundOverlay KML.