It wasn't there in my Windows 7 machine, but found it elsewhere.
I went to the Start menu and searched for regedit.exe. Once found, I right-clicked on resedit.exe and chose "Run as administrator." Clicked away the warning about allowing this program to make changes to the computer.
Once in regedit with elevated privleges, I looked under the Edit menu to use the Find... command, looking for:
ScreenSaveTimeOut
Find gave me results that, when edited, didn't change things, but doing "Find Next" a couple more times, I found the copy that I could edit and change the timeout to a more comfortable 2.5 hours (changing 900 to 9000). Editing the earlier 2 occurences didn't seem to do anything one way or the other, so I re-edited them back to their original values of 900.
My change did not take effect until after I logged out and back in.
It seems it would be trivial to make the timeout as long as you like, or to alter other nearby flags to turn off enforced login or the screen saver function altogether, but I have not tested those changes.
I am no longer under the corporate tyranny of a 15min, Group-Policy-enforced timeout! Dilbert would be proud!
Thanks for the necessary pointers! Hope this helps others!