As I am also using Geany for C development myself I have the same goal to be able to set some project settings using the workbench plugin. But this will be a long term goal and not happen in the near future I think.
Also, I still have to make up my mind how it is done best. In C the build commands are often the same but just different parameters (filepath, include directories...). So I thought about maybe extending the standard build dialog so that plugins could define own variables/placeholders (like ```%e```, ```%d```, ```%f```).