Potential oil leakage out of the side of the hub has been discussed herehttp://www.thorncycles.co.uk/forums/index.php?topic=4051.0
However, I wouldn't rule out the source of the leakage being the plug, it would seem too much of a coincidence for seals in the hub to fail at the same time as an oil change.
Your idea of repeating the oil change to equalise air pressure seems to me to be a good place to start and could save a lot of work/expense with more drastic remedies. However, you should ensure you either use Loctite to re-seal the plug or buy a new plug with sealant on it, the merits of either approach has also been discussed in a previous thread.
Jim (can't help thinking again that Rohloffs are like motor cars, they're great till something goes wrong)