There seem to be many people here saying that lemon juice IS used in their BIR madras SS (hence the purpose of this thread). Therefore, I think it's safe to assume that it DOES belong in a BIR madras from THOSE BIRs (whether you and I like lemon juice in our madras or not!) . I'm sure THOSE BIRs (that use lemon juice on their madras) don't consider it to be a "no no"!
In my opinion, for what it's worth, a madras may or may not have lemon juice in it. A vindaloo may or may not have lemon juice in it. It all depends on which BIR you go to and what takes their fancy. I'm not sure there are any sacrosanct "rights" or "wrongs" in this regard.