You said that you aren't aware of any research on DMN activity and meditation. In addition to the research showing that activity in the DMN increases during TM, this research review suggests that ALL meditation deactivates the DMN:

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4054695/

"The default mode is a network of midline brain structures, including the medial PFC and posterior cingulate, that is active during rest or when the brain is not otherwise engaged, and is thought to be involved in stimulus-independent, self-referential thought and mind wandering.96 Converging evidence suggests that meditation training may be associated with decreased DMN activity,67, 70, 87, 94, 97–99 Because increased DMN activity is associated with negative mental health outcomes,100, 101 it has been posited that “one mechanism through which meditation may be efficacious is by repeated disengagement or reduction of DMN activity.”65"

Ironically, citation #98 is one of the studies that Jim Karpen linked to that shows that TM *increases* activation of the DMN. The researchers mis-cited it to support their claim (the author of the study rolled his eyes via email when I pointed that out to him).

https://www.ncbi.nlm.nih.gov/pubmed/19862565

"eLORETA analysis identified sources of alpha1 activity in midline cortical regions that overlapped with the DMN. Greater activation in areas that overlap the DMN during TM practice suggests that meditation practice may lead to a foundational or 'ground' state of cerebral functioning that may underlie eyes-closed rest and more focused cognitive processes."