Simple solution would be to add a cheap filter to the rear 120mm, and change it to an intake, so that you'd have more intake than exhaust, and positive pressure.
That also gives you the benefit of the rear 120mm cooling the CPU socket area. That same fresh intake air from the rear 120mm is also closer to the radiator than the front fans, so it could increase performance slightly. Overall, you're suggesting a bottom-up airflow in the case. That checks out with the laws of thermodynamics.
Only caveat I see is if the case is under a desk with not much room above. That may cause the hot air to get caught by the rear fan and pump it back into the case....trial and error.
Main: i5-3570K, ASRock Z77 Pro4-M, Asus GTX660 TOP, 500GB Crucial BX100, 2 TB Samsung EcoGreen F4, 16GB 1600MHz G.Skill @1.25V, EVGA 550-G2, Silverstone PS07B
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