Changing 'Open with' setting

I'm trying to automate the application used to open files downloaded from the BBC iPlayer so the 'open with' setting is set to Quicktime player. To be clear I don't actually want to open the file, just change it as if I had gone into the 'Get info' pane.
The rule I've created is working fine but is there a mechanism I can use to change the setting? There doesn't seem to be anything built-in and I can't see how to do it in Automator.
The rule I've created is working fine but is there a mechanism I can use to change the setting? There doesn't seem to be anything built-in and I can't see how to do it in Automator.