It seems that Earth has very little trouble placing its people into extremely high positions all over in a variety of countries and societies, despite not having mind control technology - or for that matter, much of any useful technology for infiltration (they can't spy remotely, for example, requiring Rumata to be physically present to record stuff with his
Google Glass headband camera). They have their unlimited synthesized gold, but I doubt you can just buy high nobility that easily.
Don Rumata's singular case can be explained - he's a "foreign noble", in a distant province, and he assumed the identity of an actual dead noble. (and even so, Don Reba tumbled on to his cover story pretty easily, by checking that real Don Rumata of Estor is dead for several years).
How did the rest of them manage this feat? Don Kondor is far higher placed than Rumata, in Republic of Soan. The text mentions several others by name, and the implication seems to be that there are dozens if not hundreds all over.