Why are you bulk aging? Not to say it's a bad thing, but what are you trying to accomplish? That will drive the answer to this question. If you're aging to allow time to clear, that could happen rather quickly, or over a lot more time, depending on what kind of particulates you're dealing with. If this is why you're "bulk aging" I would recommend you cold crash, which is very easily done with the volume you're dealing with. Just put it in the fridge, and set the temp just a hair above freezing. I like 37F. If you're bulk aging for some kind of long-term maturation of the mead, then you might consider that a good portion of aging is allowing the slow creep of time to "breathe" tiny bits of oxygen into your mead. This is why there are different diameters of corks for bottling. Tighter corks let less oxygen in, and are intended for wines and meads to be laid down for quite a while. Wines intended to be drank while still young are typically bottled with narrower corks, or screw tops. There is more that goes on during aging than this, but my point is this: If you're looking for aging, a little oxygen in your headspace may actually be a good thing. A LOT of it, like regularly putting new air in there and frothing it up, would be a bad thing. But what you've described, I wouldn't worry too much about. If you're that concerned, like bernard suggested, blow a little pure CO2 in there to purge the oxygen, and you should be fine.