From what I understand, the theoretical saturation point of iso-alpha acids in beer is approximately 110 IBUs. Past that point you're just wasting hops because the acids won't dissolve into solution. Of course, in practice, the number you'll be able to achieve will likely be much lower. As a result, you might need to ad a lot of frozen bitter wort to hit your bitterness target. Speaking of which, you'll probably have a hell of a time hitting your desired bitterness with this method unless you go through extensive experimentation. Since you'd need to add a fair amount of frozen wort, you'd need to freeze large amounts, which (depending on your freezer) could take a long time. I'd recommend boiling your frozen wort for 10 minutes before adding it to make sure you avoid contamination.
All in all, its probably simpler to just boil the wort for the normal amount of time and avoid all these extra steps. It appears that it might save time, but in the end, may take longer and cause a string of other problems you normally wouldn't have to deal with. But thats just my thought on the concept. There are others here with far more knowledge of all things beer.