Disagree. Launch an interplanetary science mission. Two stage motor for the interplanetary burn, the first stage just barely makes escape. The bomb is riding on the first stage, it's now in something approximating Earth orbit, uses only plasma for stationkeeping. Doesn't take much stealthing for it to hide when it's that far out. Do a flyby when you want to attack.A successful launch would not be known to carry WMDs (barring other intelligence sources) until they were deployed - but when deployed, there would be no hiding who is responsible.
In hindsight it will be possible to figure out how it got there but nobody's going to be able to prove what it was ridesharing with.
For that matter, you could do this with a moon mission. Main mission never reaches escape, but the bomb flies a slightly different trajectory and is ejected.
You still have the big question of reliability, though, as well as the issue of a failed launch. Although if you'll spend some extra mass I don't think even that's insurmountable. The RTG generators on outer planet missions are packed so they'll remain intact even if the booster goes boom. The hard part is believable numbers at launch.