I have seen recipes for Indonesian Beef Rendang that call for tamarind (here is one), but tamarind is a little difficult to find here in Japan. So I have two questions:
Tamarind gives a sour and slightly molasses note to food, including rendang. To replicate it, you thus need something sour, like lemon juice, and a dark sugar flavour, like black treacle, molasses, or dark brown sugar.
For your recipe I would add 2 tablespoons of lemon juice and 1 tablespoon of dark brown sugar.
I'm not sure I agree with the addition of sugar to replicate the flavour of tamarind. Tamarind has a somewhat tart, sour flavour, it's not really sweet. It's a souring agent.
When I use tamarind, I use the dried blocks and break a part off, soak it, strain it and either use the liquid or reduce it to a thick paste.
I've also found that a few dashes of Worcestershire Sauce can impart a tamarind-type flavour without introducing any sweetness. After all Worcestershire Sauce does have tamarind in it, amongst other things so it may be a more convenient substitute.
I've used Worcestershire Sauce in a number of Indian dishes in this way with quite good results. I also cook beef rendang, but I've never used tamarind in it.
In a pinch, I've blended seedless raisins and lemon juice with enough water to get it runny and that has fooled most people.
