I can't see any difficulty in adding capacity outside peak hours. If trains run every 2 minutes in peak hours, just run them every 2 minutes when there is a surge demand expected.
If there is an issue in crew availability, then reduce frequency before the surge demand to preserve crews (e.g. if the usual headway is 15 minutes, cut it to 30 minutes while the concert is in place as long as the trains aren't full, such that there are enough crews to work trains every 5 minutes after the concert).
I'm a strong believer that more needs to be done to cater for this kind of peak but this certainly isn't as simple a solution as you suggest. It would take a huge amount of planning resource especially bearing in mind that concert and event times vary.
You need to put the spare trains somewhere when you cut the frequency. That takes time, staff and/or line capacity. Then the same when you bring them back out.
You need to make sure you've also aligned fleet maintenance planning. Just because you run a certain frequency during the traditional peak doesn't mean the stock to do it is automatically available at any time of day.
Cutting trains at 8pm doesn't necessarily 'preserve' crews to use at 10pm because if they've already started work then their working day is still ticking by whether or not they're on a train.
London Underground traditionally used this approach to run all night for new year. In effect the crews are released by running a much less frequent service than normal the following morning. However a line like the Jubilee with frequent events at the O2 and Wembley is basically timetabled with sufficient capacity for them every weekend. This comes back to what someone said up thread that the most effective approach is to increase your baseline of traffic such that events don't represent such a big percentage increase. That's why generally the South East seems to cope a bit better than regional services, because huge extra capacity is already there for commuting.