It is not possible to launch external processes. This installer executable produces false errors as indicated when installing the package.

// installscript.qs for some package
function Component()
{
}

Component.prototype.createOperations = function()
{
component.createOperations();

component.addOperation("Execute", "C:/Python27/python.exe", "@TargetDir@/script.py", "workingdirectory=@TargetDir@"); // This woks, produces no error
component.addOperation("Execute", "C:/Python27/python.exe", "workingdirectory=@TargetDir@"); // No program defined
component.addOperation("Execute", "C:/Windows/System32/pnputil.exe", "-a", "-i", "workingdirectory=@TargetDir@"); // Process failed to start: No such file or directory

No program defined and Process failed to start: No such file or directory are false errors. Both python.exe and pnputil.exe exist and it is possible to run them using e.g. command line.