That probably depends on your processor, heatsink, and the fans in your case.
My guess is that running the heatsink fan in pull will be more akin to having two exhausts and a passive heatsink. From what I understand, the heatsink fin density affects how well this works. Denser fin spacing will allow better transfer of heat from the cpu to the heatsink, but will also make it more difficult for air to flow through the heatsink.
So then the question becomes will your case generate enough airflow to "passively" cool the processor? For reference, SPCR was able to
passively cool a pentium in their mini p180.
It has very good cooling performance, better than the P182, given that its vent area is about double, and its stock fans probably capable of twice the airflow. Remember that the test system's Intel Pentium D950 — rated at 130W TDP, which is as hot as the hottest of today's quad-cores — was fitted with a underperforming, fanless Ninja heatsink... and in the Mini P180, it still provided excellent cooling.
However, the mini p180 has a large 200mm top fan and 120 rear fan, so it can generate a lot of air flow. Additionally, the Ninja Mini has relatively large spacing between fins, which makes it easier for air to flow. Still, that's and old sff heatsink so I bet many newer designs should be able to keep up. Taking into that I doubt your processor has a larger TDP than the tested pentium...
If your case has multiple exhaust fans (preferably one on the top), I think you'll be fine using the heatsink fan in pull. Probably would be better if you can find a way to fit it properly (are you using all ram slots?) though.
Give it a shot and keep an eye on the temps. If they get too high, then I was wrong.
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