For the sake of completeness, I should mention that the ResultProcessing.cfg and any other model plug-in configuration file is a PropertyObjectFile and can be manipulated via the TestStand API.
This is an advanced topic and the file structure is not documented. However, if you are comfortable with C/CVI, the source code that creates and manipulates these files can found at <TestStand>\Components\Models\TestStandModels\ModelSupport2\ResultProcessing.c.
<TestStand>\Components\Models\TestStandModels\ModelSupport2\ResultProcessing.h exports a some functions from there that can be useful. These can be called from C/CVI code or from sequence steps.
Here is an excerpt:
int DLLEXPORT _stdcall AddResultProcessingPluginToConfiguration(const char * pluginSequenceFilePath, const char * configurationName, BOOL alwaysAdd, BigErrMsg errMsg);
int DLLEXPORT _stdcall AddModelPluginToConfiguration(const char *configurationFileName, const char *pluginSequenceFilePath, const char *configurationName, const char *categoryName, BOOL alwaysAdd, DefaultConfigurationOptions *defaultConfigurationOptions, BigErrMsg errMsg);
int DLLEXPORT _stdcall RemoveResultProcessingPluginFromConfiguration(const char * pluginSequenceFilePath, const char * configurationName, BigErrMsg errMsg);
int DLLEXPORT _stdcall RemoveModelPluginFromConfiguration(const char * configurationFileName, const char * pluginSequenceFilePath, const char * configurationName, const char * categoryName, DefaultConfigurationOptions * defaultConfigurationOptions, BigErrMsg errMsg);
int DLLEXPORT _stdcall CreateDefaultModelPluginConfigurations(CAObjHandle engine, CAObjHandle thread /* for progress messages only, can be 0 */, BigErrMsg errMsg);
int DLLEXPORT _stdcall CreateDefaultModelPluginConfiguration(const char * configurationFileName, CAObjHandle engine, DefaultConfigurationOptions *defaultConfigurationOptions, BigErrMsg errMsg);
int DLLEXPORT _stdcall LoadModelPluginTypesAndConfigurations(CAObjHandle engine, CAObjHandle thread /* for progress messages only, can be 0 */, const char *categoryName, const char *configurationName, BOOL loadAddons, BOOL loadPluginTypesFirst, CAObjHandle *mergedConfiguration, BigErrMsg errMsg);
int DLLEXPORT _stdcall ReadModelPluginConfiguration(const char * configurationFileName, CAObjHandle engine, const char *configurationName, CAObjHandle *configuration, DefaultConfigurationOptions *defaultConfigurationOptions, BigErrMsg errMsg);
int DLLEXPORT _stdcall ReadModelPluginConfigurationSet(const char *configurationFileName, CAObjHandle engine, CAObjHandle *configurationSet, DefaultConfigurationOptions *defaultConfigurationOptions, BigErrMsg errMsg);