Also remember that space is big, really, really big, and that using old SLDF charts and jumping in well outside the systems normally traveled areas, you could lob in a rock from way out.
That's right. Space is HUGE. It's super easy to get in, get out, hide, etc. The great thing about gravity wells and computers is that both do the heavy lifting when it comes to setting a hunk of rock on a collision course. Fire and forget baby.
The story still works. You only have to jump outside of the common safe proximity points - common being those "typical" points with safe and decent transit times to and from a planet - and set your asteroid on an elliptical course towards the target planet. You could use a system's gravity well, but it's a slow and risky proposition. Risky because gravity wells are notorious for sucking down loose debris. Therefore they would likely be well observed. Not the best option when sending a secret, large, slow moving body towards a capital world. Even the addition of a fusion engine would only shorten the travel time, but not enough to reduce the risk of detection given the path.
The best option would be to "fire" your asteroid along the horizontal plane of a system. You wouldn't get any help from the system's star, but outside of the well's cones you'd have have more "space" to work with and less risk of detection. Either way, it's a long, long trip, even with engine assistance. The good news, is that there's no reduction in speed. No flipping end over end to reduce your velocity. An asteroid can be fired at ridiculous velocities.
Honestly, getting the asteroid into the target system isn't an issue. The issue is ultimately detection and lead time before impact.
A fast moving, non-reflective, non-transmitting object, even with a lot of mass would be hard to track unless it was super close to orbit. Radar isn't much help unless it's close to near-earth orbit. At which point you're already SOL. What you're really talking about is visual detection. And no offense, but there's no friggin way Taurus is tracking 100% of their near orbit, or beyond. Even if they were, visual detection is short-ranged. Again, by the time you saw the asteroid you're already SOL. A neutrino detector is absolutely worthless in a system with heavy traffic, and again, would require a general line-of-sight to be useful - meaning, you'd have to know where to point the bloody thing. If the strike is timed to hit during any type of celestial event, tracking becomes even more difficult. So if the Cluster is known for any type of common eclipses, flares or other events, they could be used for additional cover. IF the working hypothesis that the Blakists added some stealth tech to the Asteroid you can start multiplying the difficulty rating even more - though any type of electronic transmissions, outside of something hyper-local, say around the fusion engines, would be detrimental to the asteroid's innate strengths.
Remember that the size of the asteroid, if composed of dense materials, doesn't have to be a mile long to detonate like a nuclear weapon.