Provide the commandline params for run method as well

Is there a reason why you must use TestedApp?  Can you use the WShell object to execute a command line for your application?  Or set your tested app to run a batchfile that you edit and save at startup?

The reason I say so is that I'm not sure how feasible it is to write such an object to disk at run time.  The run-time TestedApp object is exactly that... runtime.  It exist ONLY in runtime as an in-memory object for the app that you're executing.  What you're proposing is editing the XML formatted file that is read into memory for that runtime object.  While it probably can be done, it seems like a problem with the current TestComplete execution architecture... much like altering script code or keyword tests at run time.