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Ferrofluid does not need a "sealed gap". It is held in place by the magnetic field in the gap. Venting is sometimes/almost always needed with woofers. Because the air displacement can blow a significant amount of the fluid out of the gap during high excursions.
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That's right, exactly. The point I was making is that some mechanism needs to be employed to deal with splash, especially in a subwoofer application. You have to seal it some way or the fluid will separate and leak away. Splash guards, alternate venting, something - The large gap and long excursion of a subwoofer make ferrofluid leakage a problem.
I imagine leakage is why there are more hifi tweeters and mids with ferrofluid than there are high-power subs that use ferrofluid. It seems like a good solution to me, but the leakage problem is probably the hardest one to solve. Eminence told me that's why they don't use ferrofluid - Too much leakage. I forget if I heard that from Jerry McNutt or Chris Rose, but I do recall talking to them about it in the context of an upgraded LAB12 driver I was looking at having them make for me a couple years back.
