Not sure how, since the optimal gameplay loop would be to curbstomp mobs that are relatively easy. It'd just take longer.If you want things to be harder simply because of level differences, returning to the 2000 xp tables, with Hell Levels fully implemented would do more than simply locking everyone to level 45 for a day.
A successful level-locked server would have the hardcores progressing through increasingly difficult content as they gear up and get new levels at regular intervals. 1 per day is too fast for most, something like 5 levels per week until 25 and then 5 levels every two weeks until 50ish makes more sense to me, but the premise is the same whether it's balanced for poopsockers or casuals.