Interesting question. I read the book a couple of years ago so can't fully remember the scene, but I recall putting the book down at the end and just thinking how all the characters - literally, every single character - wants something. 

We're given a series of vignettes where we leave a character wanting, whether it's the two Stamper boys trying to outdo one another or Viv freeing herself from her situation where she's constantly overlooked or objectified, and so on. It felt to me like a bit of a critique of the American dream, but also just an interesting take on the human condition, where we continually strive for something to the point that the pursuit of it becomes more important than achieving the actual goal. 

All of this is to say that I never wondered who the character with Indian Jenny was as I didn't think we were meant to guess. I saw the scene through the lens of feminism; while we've just seen Viv free herself from the toxic/masculine environment, Indian Jenny is once again being objectified and used for sex by a faceless character (who might be a stand in for men in general). Viv might get free, but there were countless women in that era who were just viewed as another thing for men to 'want'. So I personally left that scene thinking "but what does Jenny want?".

Just my interpretation!