[Micah]

Not auto, but you can group the hunters, select them, and then hit "z" to lab-dump them, so it's one click and one hotkey per blood prov which isn't too bad.

Edit: Or hit "pool slaves" from the gem menu, though this will cause any blood sacrificers to need to be restocked manually, which is more of a pain than using the z shortcut if you're doing much blood sacrificing.