According to this Google Policy:
Publishers are permitted to make modifications to the AdSense ad code so long as those modifications do not artificially inflate ad performance or harm advertisers.
Loading Ad. script on Window load event in no way breaches this agreement. So by this one sentence alone, you can be sure that it is allowed.
Also, according to this Modification of the AdSense ad code document:
In general, we recommend copying and pasting the ad code. In some situations though, we understand that modifications are crucial to a clean user experience.
So here as well you can see the same confirmation. Basically as long as you don't fundamentally change how the ads are being displayed, you are fine.
Furthermore, in the Techniques to avoid section, you'll see none of the points conflicts with window load event. On the other hand, the Acceptable modifications section clearly states:
Here are some acceptable modifications ...
That means, not all the acceptable modifications are listed there. So, as long as you simply load the CODE in window load event & do no further artificial delay, Google have no reason to penalize you, since window load event is one of the most common Browser Default method people use to load additional scripts and contents. At best they may say that it's not necessary anymore & their asynchronous CODE is optimized enough.
Finally: I've done this myself on my own site and other client sites & never faced any problem whatsoever. Also didn't ever heard or seen anyone having any issue for this. Even a quick search on Google didn't reveal anything like that. So you should be fine.