Finally I have had some time to implement the functionality I was requesting. I am attaching an explanation on what the motivation was, and how it can be used.
Since I am not at all an expert on C, neither on the rocrail code, it has taken me longer than expected. Moreover, there is still a problem with the code – a memory leak – I have not been able to solve. I think it is in the way I am attaching the speedcurve parameters to the loc command (loc.c file)… but I am not sure – any help, correction and suggestion for improvement is very welcome.
Basically I have modified the wrapper description file to include a new property to the loc (speedcurve). Then I have modified the loc.c file to explore if a locomotive has this property and include them in the commands sent to the stations. Finally, in DDX, this parameter is treated in MM locomotives.
The reason to address only MM is basically because DCC decoders use to have an actual speedcurve functionality. In any case, since the information is transmitted to any station, anyone could make use of it to implement the speedcurve as I have made for DDX MM.
Hope this new feature to be of interest. It needs still some refinement for the memory leak.. but hopefully someone could give me a hint on how to solve it, so it can be included in a stable release – if it is of interest of course.
I am attaching the three source code files modified, a plan example and the explanation of the functionality.

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