Anviloy will certainly stand up to the temperatures of aluminum. It might not even require a copper heatsink to dispel heat.
My worry though, is this, aluminum tends to shrink. If your core is ever in a position that's surrounded by aluminum, with gravity working against it, the core could become frozen or stuck in the casting. My steel core certainly did.
I too am building a steel die. Mine is for gravity casting. I'm going to attempt to mix a sand core with a steel die. I'll update this thread after a test. Hopefully within the week.
For more information on sand cores, search within this forum.