Adam,
I don't have a lot of information on T. H. Riggs-Miller, but I have come across his name a few times.
T. H. Riggs-Miller was associated with Peterson, Sinclaire and Miller, Inc. prior to June 1916. This company was responsible for implementing the Carters system in the States. Peterson, Sinclaire and Miller, Inc. consolidated with Carters Tested Seeds in 1920. In a June 1916 publication of The Golf Course, there was an announcement that T. H. Riggs-Miller is no longer associated with Peterson, Sinclaire and Miller, Inc. I am curious whether the "Miller" in Peterson, Sinclaire and Miller, Inc, referred to T. H. Riggs-Miller? Peterson was Arthur Peterson and Sinclaire was R. O. Sinclaire, but I have never found evidence of the name behind Miller.
In the late 1920's and early 1930's, T. H. Riggs-Miller contributed several articles to Golfdom. His articles dealt mostly with greenkeeping techniques. During this same time period, he was the greenkeeper at Richmond County Country Club in Staten Island, NY. The Golfdom articles can be found through the MSU Turfgrass Archives website.
In 1935, Riggs-Miller was reported designing and laying out an 18-hole course for Lawrence Country Club in Lawrence, Kansas. The article mentioned that Perry Maxwell would be responsible for the construction of the course.
In 1940, T. H. Riggs-Miller passed away at the age of 52 in Kansas City, Missouri. The short entry in Golfdom mentioned he had come to Kansas City several years ago to redesign the Swope Park Course No. 1.
These are the few references I have found for T. H. Riggs-Miller.
Bret