Alas, a quick test shows it doesn't work. I verified the call to setSafeFrameConfig, but the returned ad still shows up in the middle of the screen.
Can you try also setting the allowPushExpansion property to false:
Code:
googletag.defineSlot('/12345/ROSScraper', [[120, 600]], 'div-gpt-ad-12345').setSafeFrameConfig({allowOverlayExpansion:false,allowPushExpansion:false}).addService(googletag.pubads());
Unfortunately, the documentation isn't clear exactly what push expansion is compared to overlay expansion. Sometimes, we have to try the brute-force method of testing.