Will a hot cornstarch slurry added to a cold mixture still have a thickening effect?
It would probably work but for the proper consistency of the finished product, your cooked cornstarch slurry would need to be very thick. This would make it difficult to mix into a homogenized smooth mixture unless you were to use a blender to mix them.

Rather than cornstarch, xanthan gum or arrowroot flour, why not use tapioca powder/flour? It thickens quickly starting around 140°F (60°C) and gives a nice clear gel, especially nice for fruit desserts. It also holds its gelling property and won't separate like cornstarch based desserts can.

I buy my tapioca flour in an Asian grocery store but I've also seen it sold in the Asian foods aisle in many larger supermarkets. It's quite inexpensive.

You substitute the same amount of tapioca flour as cornstarch in a recipe.