A substitute for jaggery
Traditional white sugar is pure sucrose...Molasses, which is the difference between brown and white sugar is also pretty much pure sucrose (at least the sugar part is). But according to your link, there is a significant amount of inverted sugars and water in jaggery...That sounds more like honey to me, and it'd be a pretty significant change in the chemical behaviour.

I don't think you're going to get a direct substitution, but I think a combination of some strong honey, white sugar, and blackstrap molasses might get you a bit closer...Blackstrap molasses is the most strongly flavoured molasses, so use sparingly, but I think some of it's odd flavours might add to the complexity of your sugar mix.