Hi Luigi,
when you just type "mess engine_name" manually
a) picochess chess won't notice and know that running engine
b) you have to run it with the plugin option otherwise you won't get the uci wrapper
So the trick was to modify the picochess.service described by Kiko that allows picochess to have these engines run with gui output and you have to modify the engine scripts for running with graphic output.
It is all somewhere "hidden" in the image thread but I think Al might be posting new consolidated instructions soon (the basic tings I mentioned above will be the same for desktop/VNC Client and XRDP methods)...
Dirk