To get a better sense of this question, you may want to refer to game 5300 (Turkey and Italy), Spring of 1907.
Turkey 'requested' that Italy assist in the convoy of a unit from Rome to Greece. Italy was located in the Tyrrhenian Sea at the time. Italy 'agreed'. But given the two nations' history during the game, Italy would have much rather slit Turkey's throat (or its own) rather than be of assistance.
Why is it not possible for Italy to agree to the convoy, but rather than actually sailing to the agreed upon destination to hand over the troops within her holds to the Turkish fleet in the Ionian, to either drop the Turkish troops over the side into Davey Jones' Locker, or at least strand them elsewhere (so long as this elsewhere were somewere that Italy could readily access)?
The way the game is set up, because army being convoyed defines the final destination, rather than all the intermediate steps to get it to its final destination. This could have implications in that a convoy could fail at any stage. Were a convoy to fail near the tail end of a multi step convoy, the forces in the fleet's hold could disembark at any unoccupied, readily accessible territory closer to the final destination, rather than remaining at its point of origin.
In the above example, Turkey defined the move of Rome to Greece via convoy, and the only possible monkey wrench open to the Italians was to fail to show up for the pick up. Which is what the Italians elected to do.
Abydos1 - When picking up the army at Rome, you smile, ask for their documentation, orders, etc, etc, etc and say "But of course we are taking you to meet up with the fleet in the Ionian Sea." Their own strategic command believes this to be true too.
They board. You put them all below deck, lock the doors and then sail off. If you had to abandon your own ship and sink her to achieve your end, so be it. Sounds realistic to me - far fetched maybe - but definitely feasible.
you've got an army on your ships, it is going to kill all your sailors when they try dropping them into the sea or in the wrong place, at best both units will be destroyed.
Otherwise, you can get a paradoxical situation which is not resovled by the original rules where an army being convoyed would (if succesful) cut the support of a unit which is supporting an attack on the convoy route.
Also if you are not forced to specify whether an attack is via land or via convoy the cunning players could order a fleet in adr (say) to convoy trieste to venice, and since Trieste order his army there to venice it goes through. Meanwhile venice is ordered to trieste, and also gets through because the other army was convoyed around.... possibly very useful, but not allowed in most versions of the game, because highjacking is considered silly.