Any question answered...

My karrimor rucksack came with a rain cover that seems like a simpler version of the hump (plain black). It probably wouldn't stand up to hours of persistent downpour as the rain could still seep down your back or along the straps and slowly soak the bag, but i've found it works pretty well for keeping things dry when cycling in the rain.

The advantage over putting waterproofing inside is that you don't arrive with a soggy rucksack.

humm interesting. might end up biting the bullet and getting a hump. maybe i'll start undertaking too, could be fun