Slugging doesn't seem to really exist on a sustained basis anywhere other than NoVa. My guess is that it requires a specific set of circumstances to take off: SOV commutes need to be generally feasible (so that you have drivers), HOV lanes need to exist (so that drivers have a reason to pick up passengers), you need specific job loci (to make driver/passenger matching exist), and you need commuter buses (to provide the passengers).

The Pentagon and I-395 provide one of the most optimal conditions for slugging, with the added bonus that you have effectively direct access ramps from Springfield and the Pentagon that completely bypass the regular lanes. Since I-66 was HOV-only during the rush hour until recently, that probably retarded the existence of slugging despite HOV lanes (since the SOV backup wasn't so feasible). Similarly, the lack of HOV lanes in MD (save for 270) also retard the impetus for slugging.