According to at least one Kalua recipe that I've found, you can just wrap in aluminum foil instead. That would imply, to me, that they're not used for any (significant) flavor. The recipe does use a single banana leaf for flavor, but Ti leaves for the outer wrapping. Using foil may not give you the exact same results, but it might be "close enough" for people who don't actually live in Hawaii.
Saltpeter is potassium nitrate, which does not directly cure meats. Bacteria convert nitrate into nitrite, which is the real preservative. Saltpeter can be replaced by a smaller amount of nitrite to get the same curing effect (most commercial cured meats do this), though a prolonged cure that converts nitrate into nitrite can develop more flavor.
Tender Quick is not a direct substitute because it contains mostly salt. I've heard that you can replace the salt in your recipe with Tender Quick, and drop the saltpeter, and have a success. You would have better luck finding a recipe that was meant to use Tender Quick, though.
It is definitely possible to buy (food-grade!) saltpeter. I would check online, or at specialty stores. It's a little more difficult than picking it up at your local grocery store, of course.
(Chemistry lesson, courtesy of McGee: nitrate (NO3) is converted to nitrite (NO2), which then reacts to form nitric oxide (NO), which bonds to myoglobin in the meat, which turns it pink and prevents oxidation. Nitric oxide is also present in smoke, which gives that "pink ring" around the outside of smoked meats.)
Best Answer
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.