This might be a job for the custom vulnerability feature of the security and patch. No need for SWD. You can set it to Autofix and whenever the reg settings are changed the custom vulnerability will kick in and change them back. very powerful and allows a hands off approach.
Eric is correct and a custom vulnerability is the way to go. Also no need to script if you use the assoc command - http://ss64.com/nt/assoc.html So if you don't want to use Patch create a simple batch file:
ASSOC .tif="MSPaper.Document" ASSOC .tiff="MSPaper.Document" ASSOC .mdi="MSPaper.Document"