I have a paper map with survey points. They used a custom graticule and origin, and included a site register which lists each site's location according to this system. This was all done to preserve data integrity. My end goal is to visit these locations in person/view them in Google Earth. I know the original projection of the map with which they recorded locations, and also the coordinates of their origin point.

While georeferencing is an option, I am wondering if there is some way to use Excel, etc. to convert these coordinates (i.e. 117 N 140 E) into usable coordinates (WGS decimal degrees for example).

I was thinking about making a custom grid, digitizing the points by hand based on the grid locations, and then reprojecting the points. Would this work?

If I understand correctly, you can create a custom coordinate system using the original projection as a starting point, and change the false easting and northing parameters to account for the offset.

Once you've got the points entered and defined by your custom projection, you can Project the points to whatever standard coordinate reference system you'd like.