One potential option that could be used: We currently have the "is" and "is not" options available to us in GPP. What you could potentially do is use the "is not" condition to apply the appropriate value. Assuming that the ceiling/max is 15 minutes, if we assumed that most users would potentially set the screen saver value to 5 minutes, 10 minutes or 15 minutes, one could add the following items:
"the string register value at HKCU\Control Panel\Desktop\ScreenSaveTimeOut does not exist or does not have value data 300" (this would be 5 minutes)
AND "the string register value at HKCU\Control Panel\Desktop\ScreenSaveTimeOut does not exist or does not have value data 600" (this would be 10 minutes)
AND "the string register value at HKCU\Control Panel\Desktop\ScreenSaveTimeOut does not exist or does not have value data 900" (this would be 15 minutes)
If none of the conditions apply, then reset the screen saver value timeout to 15 minutes in GPP. You can add more condition cases to suit your environment (for instance if you wanted to also allow 1 minute screen saver timeout options).
From a processing perspective, if the user chooses the default value of 15 minutes, then processing will stop after the first line since ScreenSaveTimeOut would be equal to 900 (15 minutes*60 seconds) and hence the first line would fail short circuiting the AND condition (I am assuming that GPPs use short circuiting).