I assume you are talking on PlotReactorManager's events, such as Begin(End)Plot/Document/Page...
As Tony pointed out, you cannot run another command (by "SendStringToExecute()") because of "Plot" command being executed. However, it does not mean you cannot do something in the event handler with your code. Just do not call SendStringToExecute() for a command, built-in, or your "CommandMethod" defined command.
Depending what you want to do, writing some code in the event handlers may be much more simpler than defining your own "Plot" command. For example, I use BeginPlot/Document to keep track of plotting work done from AutoCAD, including user who does the plotting, plotter name where the plotting is done, plotting paper size and copies... All the tracking data go to a database. There is more you can do obviously.