Since you have to leave it in for so long, have the roast be somewhat icy (not frozen solid, just icy) when you put it into the crock. Yes, this is completely safe to do. Also, don't cover it with water. You are roasting it, not braising it. A small amount (1/2 cup) of liquid in the bottom of the crock is more than enough. If you can, set a timer to have the crock come on a couple hours after you load it, that will help reduce the total cooking time. Remember, since the roast is icy it will remain at safe temp until the crock comes on. And, if your crock is programmable, then by all means have it switch to warm for the last hour or so before service.