Mattias, www.hornusa.com or www.phorn.de should be able to direct you to their keyway broaching tools. They cut by stroking a broach insert tool into a hole. I suppose you could come up with a method to broach externally as well. Good Luck
add: The horn site is a bit confusing. click on their site then products/catalogs/non-rotating/broaching/ open pdf/ scroll down to info you want.