I'm not so much interested in arguing whether they work, or should work. I'm just happy that I found their literary inspiration. Or one of them, at least.
I have parts of a multi-volume set of Poul Anderson, the great sci-fi/fantasy writer. It includes "The White King's War", a Dominc Flandry story. Flandry finds himself shipwrecked on an inhospitable planet (i.e. no booze & no nightclubs to be found) so he tries to contact the Empire for a rescue. He uses these:
"The gadgets, four in number, were built as simple as possible. Inside a torpedo shape - a hundred and twenty centimeters long but light enough for a man to life under Terran gravity - were packed the absolute minimum of hyper-drive and grav-drive machinery; sensors and navigational computer to home on a pre-set destination. radio to beep advance notice when it neared; accumulators for power and a tiny space for the payload, which could be a document, a tape or whatever else would fit.*"
Standard CT turret missiles are 50 kg, which while heavy could be moved by one man. TTB does not say how long a missile is, but 120 cm seems reasonable. So, this might also be where the designers of Traveller got the specs for 'normal' missiles.
I do not know if this is the only time in the Flandry stories that these gadgets make an appearance. It turns out that the message torpedo does not make it even to open space, so it fails to communicate the SOS, and Flandry has to find another way out of his predicament. Taking that unreliability into account, I'm not sure that having them would upset the balance of Traveller. No one uses them as regular communication channels, they are meant as a last-chance call for help.
To program a message torpedo to summon help: 8+, DM +Navigation 1-2 hours. Referee makes the throw in secret, the PCs will not know if the torpedo has gone off-course or not.
To re-purpose a turret missile for sensor or drone operation: 8+, DM +Electronics or Mechanical; 3 hours. A failed roll either takes longer (miss by 1-2) or ruins the missile (miss by 3+)
* The White King's War, from The Collected Short Works of Poul Anderson, Volume 5: Door to Anywhere. p 180.