I don't see why we can't treat this similar to the other two requirements (internet and power supply) with the caveat of offering only the manual partitioning option. With that being the only option offered there should be room to add an ample warning.
Please see:
https:/ /bugs.launchpad .net/ubuntu/ +source/ ubiquity/ +bug/766265/ comments/ 23
I don't see why we can't treat this similar to the other two requirements (internet and power supply) with the caveat of offering only the manual partitioning option. With that being the only option offered there should be room to add an ample warning.