You can't do it now but I know a team that promoted out of USA division V last season with just 3 real trainees and a bunch of scrubs he bought for $1k each week. He then fired them and so at the weekly update only had to pay the wages of the 3 trainees. With the mountain of money he saved he bought a couple of real players for the playoffs and won. It is that easy.
I thought the rule change was just that you couldn't relist them until four days have elapsed, but firing was still viable. I think the bigger detriment to this idea is the salary floor, unless the three trainees' salaries reached pretty close to that. (Now that I check my league, it's only $32,800, so I suppose it's not unreasonable).