Kidnapping Lucius and Narcissa would be extremely difficult and holding them prisoner even more so.

I might be able to put Lucius into a vulnerable position and let the Order know when to show up, but I don't have any sort of access to Narcissa, and MLE keeps an eye on her -- not ordinarily because she's a hostage, mind you, but simply because she's the wife of a powerful man. And we would need to take them both before either were missed -- I don't think Draco would thank us for saving Lucius, if Narcissa paid the price.

You'd have to take them to Saltash. Even that might not be enough.