Spraying with a strong jet of water sounds fun (do you think a water pistol would work for those with allotments without hoses?) but I would worry about blasting the tops off my brassicas with over enthusiastic spraying! Sadly I am unable to encourage the aid of small birds in this because I have to net my brassicas to protect them from pigeons and deer.
Perhaps one could encourage spiders? Presumably they would love to eat mealy aphid. Are there any other natural predators that could be persuaded to join in?
When gardening under the nets, to squash slugs or tidy up leaves, I always think the whitefly makes the plants look haunted, the puff of powder and small flies when the plants are disturbed is slightly sinister, so spider webs would certainly add to this effect!