Is there anything I can do short of leaving a glass out on the counter to mitigate overcarbonation? I recently picked up a bunch of bottles of what seemed like interesting brews from Hitachino, because I was interested in exploring some Japanese beers. Unfortunately, the first two bottles I've opened now seem to have both been severely overcarbonated.

We're talking foam overflowing out of the bottle once I open it. These bottles have not been exposed to significant agitation recently. They've been sitting on a shelf, upright as is proper, for days. Worse still, they don't just explode and make a mess, but they also simply taste severely overcarbonated. It's like drinking shaken seltzer water mixed with my beer, and it's decidedly unpleasant. At this point, the only thing I've been able to do is pour a glass, leave it out for an hour or three, and then drink it.

Which is... impractical at best, and is only going to become less so as the weather warms up. Is there anything else I can do to mitigate this?
You can decant the beer between two large glasses or pitchers - the agitation will cause the CO2 to come out of solution quickly and also not raise the temperature too much. 

Sample after 4-5 decants to see how much the carbonation has dropped, and repeat as necessary. You will end up with quite a bit of foam, hence the need for larger glasses or a pitcher.