I think it's a very good idea. Otsukare thanks.

IMO, some improvements will make it even better.
1) Separate from the convoy status message and assign a dedicated shortcut key. Because it's different genre information than displaying convoy alerts.
I don't think it is a good idea to hide the anomalous status with this display. Not suitable for constant display.
And while this display is convenient, at the same time it tends to be very annoying, so it is preferable that it can be easily turned on and off. However, it is troublesome to switch if it is in an unrelated cycle. It would be convenient to be able to separate and turn on and off only that.
2) Add an option to show it only when the mouse is over.
As mentioned above, this display can be very annoying, so if it has such option, I would prefer to use that option during usual.
3) Avoid confusion with station name displays.
The station name can switch between multiple display styles, but this is not possible. Therefore, if the station display is a square box with a player color background, it is difficult to distinguish it from that. (I usually use that style.)
I had some ideas; adding a waytype symbol to the name, adding a status bar to make it look different from the station etc
For the time being, I try to avoid conflict with the station display by making white letters on a dark background.
4) Make a distinction between line name and convoy name. I tried changing the background color slightly, but it may be difficult to understand. The background color of lineless convoy is a little dark.
I made a version with these improvements.

The repository is here
https://github.com/Ranran-the-JuicyPork/simutrans-extended/tree/independent-convoy-nameplate# TOOL_CONVOY_NAMEPLATES=133
simple_tool[133]=,0
We can assign its shortcut key in menuconf.tab. In the example, it is assigned to the "0" key. Well this is a pakset task. In Britain-EX, the key of the convoy status tooltip is assigned to shift + 7, so a key close to this is preferable.
Future improvement idea:
Added a symbol for whether it is a reverse route on it. For example, an up arrow and a down arrow.
↩ ↦Aside from that, your original code does not correctly correspond to key assignments. If you implement it with the original specifications, you will need to modify it by referring to the prissi's code.