Personally I like piercing the tongue, like Cicero, a little less messy.
This is actually a really good way to deal with this. If they have had their tongue pierced by a cold iron implement (nail, awl, etc) then Speak with Dead will not work on them. It also sets up possible evidence for later.
This way you are being respectful of the world rules but also acknowledging that criminals have evolved to get around them.
