This is an interesting discussion. I have entertained fleeting thoughts of implementing some element of convoy modification in Experimental, but consider it a low priority and have not looked into it in detail.
I had thought to implement it in a somewhat generalised manner, which would allow for things like dividing and combining of trains, slip carriages, locomotive changes as well as the pick-up goods sort of operation considered here.
Stations could have a "sidings", "shunting yard", "stabling shed" (etc.) type of extension. This would be indicated by a simple boolean flag in the station extension object and a "shunting=1" or the like indicator in the .dat file.
At any station with the shunting type of extension, orders could be set on a convoy's schedule for the modification of its vehicles. The station would, like a depot, keep track of the vehicles dropped there, and they would be accessible by a display, and later convoys could pick them up. Vehicles could be sent straight from a depot to a station with a shunting facility and left there to join a later convoy. If not enough of the right type of vehicles are present when the convoy comes to the stop, it can either, depending on the setting in the schedule, wait for the correct number indefinitely, wait for a certain period of time and then continue regardless, or continue regardless without waiting.
I suspect that this would take quite a bit of time to write, and would be rather fiddly to operate, however, as players would have to synchronise quite a lot of things to get it to work properly.
One challenge that would have to be resolved is this: how reliably to make a convoy shorter or longer other than in a depot? If one just adds vehicles to a convoy, especially a railway type convoy with block sections, how would it work? What if it is in a platform of restricted length and the new vehicles added make it too long for the platform? What if the new vehicles make it too long for the block section such that it sticks out of the back and occupies two rather than one section? What would happen to any trains in the rear section at the time, or that had reserved the rear section?
The challenges in the foregoing paragraph, incidentally, are also applicable to a much higher priority project on which I am working, to enhance and simplify the vehicle replacer in Experimental such that convoys using it no longer have to go to the depot; if anyone has any good ideas as to how to overcome these issues, I should be very interested to know what they might be.
Similarly, if anybody considers this implementation of adjusting the vehicles in a convoy during its schedule to be worthwhile and a higher priority than my other
coding projects, do feel free to write it on a Github branch and I shall certainly give serious consideration to including it.