It's an interesting concept, but it strikes me that the bazillions of $ we'd have to sink into this isn't really worth it, considering the odds we'd ever use it in anger. If it isn't worth a million-dollar Tomahawk to take out a cave, why would it be worth a $500M ballistic missile? And that's assuming the intel is considered solid enough to launch the thing in the first place.
Now, as a conventional deep-penetrator (giggity) that would give us the ability to knock out a deeply-buried bunker or facility in a well-defended country without sending manned aircraft in...I could feature that.
I don't think the how-will-Russia-know-it's-not-a-nuke thing is as big a deal as some are making it out to be. The danger in a launch like this is that China or Russia will think it's aimed at them and retaliate, right? But a single launch - which is how we're talking about employing this - isn't likely to freak anyone out. You can tell from the ballistics fairly early in a launch roughly where a missile is going to land. And a single launch doesn't really imply a sudden first-use nuclear attack, which is what would startle the strategic nuke powers into retaliating.