I don't have quite enough Belgian Dark Candi Sugar Syrup for a recipe, can I replace a quantity of it with Treacle?
One could always make up a solution of various sugars to make ones own syrup. or one could mix golden syrup and dark molasses/treacle to make a custom Candy syrup near equivalent mix. Some brewers warn against using too much treacle as it can impart a "licorice" flavour. Personally I prefer to use maple syrup as it gives a nice, slightly caramel flavour to the brew. However the precise answer depends on what style of beer one wants to brew. Some styles are more demanding than others, but I have never found any sugar syrup produce a bad brew. IMHO syrups tend to increase the ABV but not affect the final brew flavour to any great extent.
The D-180 not only adds sugar, but also colour by way of dark caramel. If you replace this with golden syrup , then you will get the sugar, but not the colour. If you would use the same amount of black treacle then you will indeed have the liquorice taste (which you might or might not like). Even using an amount of dark Belgian sugar (like from Tienen or Graeffe) gives a liquorice taste.