Mental contrasting is one of the more tested useful interventions in psychology, including in field studies intended to help actual people with their actual goals. (Though in field studies it's often paired with implementation intentions, an even-more-tested intervention, to give people a double dose of help, which makes those studies less informative about the benefits of either intervention.)

The papers cited in the op-ed, and many others, are available on Oettingen's website. One of the papers is a 2012 review article summarizing the research; it's rather long but you can get a decent picture by reading all the section headings (and then diving into whatever seems interesting):

Oettingen, G. (2012). Future thought and behavior change. In W. Stroebe & M. Hewstone (Eds.), European Review of Social Psychology, 23, 1-63.