I recently modified my Blichmann conical to include a bulkhead-mounted, Corny gas-in post so that I can add CO2 to the fermenter during the closed transfer to keg. Using my 5 lb CO2 cylinder, its a piece of cake to dial up a trickle of CO2 to keep the blow off from sucking air into the fermenter.
Several years ago, I changed the air-lock on this fermenter to a blow-off tube inserted into a clear tube. So I can anywhere from a fraction of an inch to over a foot of water column pressure inside my fermenter. The side benefit of that change is that it takes a much larger volume change inside my fermenter to cause air to be sucked into the fermenter. As you know, it only takes a teeny volume change for the typical air-lock to suck air. So if you haven't done this, converting to a blow-off tube is a really good thing to do for preventing air ingress to your fermenter headspace.