Wow, Empty Hex Jumps, Jump Torps, and near-c rocks all showing up at the same time here... it's all the classic Traveller flame wars in one place! Who needs any other boards now...
I don't buy the "if it was possible it would have already been done" argument. I guess far-trader missed all those broken planets leftover from the Ancient War? (that said, those need a MUCH bigger explosion than just a big rock hitting them at lightspeed to be turned into an asteroid belt)

.
And frankly, there's nothing stopping you from doing it in the OTU - it's a consequence of a poorly-thought out technology with unlimited energy that has no practical limitations. Take one planetoid hull, add 6G drives, load it up with fuel for the powerplant to accelerate using reactionless drives for as long as it likes (since plants bizarrely use fuel at a constant rate regardless of how long they're operating for or how hard they're pushed) and you can accelerate up to any speed (limited by relativity, but you can get to a good 0.8c at least before you even need to worry about that). Aim at planet, and blow a big hole in it at the very least.
Someone will have tried it. Someone will have succeeded. Maybe everyone saw the consequences and though "whoa, we're never doing THAT again" (like the way that nobody (so far anyway) has nuked another population centre after WWII). But enemies (internal and external) might be up for it, and it's really easy to do and to set up, and by the time it's detected there'd be no hope of evacuation from the target.
But while there's moral reasons to not do it, there's absolutely no practical reason whatsoever stopping anyone determined enough from dropping a rock at bloody fast speed (at least) onto a planet.
Generally, I think one big problem is that reactionless drives don't use fuel - which is one reason I liked TNE because it dropped them in favour of HEPlaR, which DID use fuel and thus limited what ships could do.
Another problem is that you're moving a few million tons of rock at the very least - which probably doesn't even have the structural integrity to hold itself together under any kind of acceleration (i.e start accelerating most asteroids, and you'll just break them up into the rubble piles that they really are).