Well, the sarong looks East Javanese to me. Of course that is no assurance of the origins of the blade itself. No doubt you will still find communities of Balinese in East Jawa, but i would not compare this situation with Bali and Lombak. If i am not mistaken, the Balinese court actually once ruled over Lombak. This would not be the case in Jawa. That would have a very strong influence of the type of keris made in Lombok. That same Bali court influence did not exist in Jawa. Keris throughout Indonesia certainly are related and you are right to point out that at times it can be difficult to determine origin. I will take your statement one step further and suggest that unless one has strong provenence for a keris (which is rare) one can never know for sure what mpu made it. It is generally much easier to tell what area a keris is from. Still.....sometimes not.
I asked you the question because to me this appears to be a keris form well known to Jawa that was presented in East Jawa dress. The surface of this keris appears to be rough from etching, not polished as i understand is the tradition for Bali keris. I suppose it could be a Bali keris that has been kept in a Javanese manner, but i would just be guessing there. So i asked the question why you thought it might be Balinese because i thought you might have some substantial reasoning.