The Liege syrup is made by cooking apples and pears down to a thick concentration. The end result is dark brown paste. I would also add pureed raisins/dates/figs to the prune paste. Will get the Belgian paste and make the meatballs next week.
Sour tends to go well with fatty or rich meats. In Germany you eat blood sausage with apple sauce. Apple sauce is versatile, it is to cut the stodginess (and to me, also enhances other ingredients). You even eat it alongside alpine mac and cheese in Switzerland. Roasted pork with apple sauce is a given.
Pomegranate molasses or juice is great with duck and goose. I have braised ducks in it. Remember to check the content of pomegranate concentration. Iranian has the most.
I have seen 2 recipes using grapes. The French one is a braising dish and the Austrian one is with Sauerkraut. I thought Sauerkraut with grapes were too much sour… but maybe it has something to do with grape harvest time in a wine producing region in Austria