Ok, I hired King to dig through the DDL archives and he found my old rule about this situation.
Because that ^ is on the record, it HAS to apply to the current situation.
This is just such a rare situation that everyone forgot about it. In most weeks (95%+) owners don't even reach 5 adds. It's so rare for a playoff team to have a good, addable prospect during the playoffs that it simply has not come up.
The fact that this has arisen exactly once in between that rule being stated (March 31 2014) and now demonstrates that this issue is exceedingly minor. Accordingly, the rule will not be altered because it is not a pressing concern.