Topic on Talk:The big list of third-party DRM on Steam
The patch installers (at least those inside DLCs...) does not replace masseffect.exe. Otherwise the game wouldn't even launch (and I would have noticed d:) But the DLC installers are coded in such a way that if they detect the Steam "flavour" key in the registry, they won't run that
My observation was referring exactly to the original Steam executable. Anyway I checked it with that nice tool (PROTECTiON iD) and it only reports steam protection. So.. I think it's fine.
Even though it's strange that Steam .exe is still 20MB big (whereas in GTA:San Andreas for example, securom removal slimmed down the exe from 15 to 5MB)