How can I prototype an Aluminum-clad board?
If you're willing to live with design rules like the cheapest aluminum core board (single sided, with no plated through holes) you could buy the thinnest prototype board available (eg. 0.4mm) and simply laminate the blank side (or make it all floating copper) to a sheet of aluminum. 

There are companies offering to prototype aluminum clad boards though, and I would expect the cost to be reasonable (from your perspective) if it's something that will eventually go to production.