I've had a look at the preapproval API which looks kind of like what I want but not quite.
I need to mimic the way Kickstarter works fairly closely; users are sent to the Paypal site where they sign in and agree a payment then on a certain date the payment either gets taken from their account or, if some conditions aren't met, nothing happens. I know Paypal are 'funny' about some uses, but actual goods and services are for sale so I think it should be OK.
I need many people to be able to contribute to one larger payment; I don't know if there's a way of setting up a something like this inside of Paypal itself or if all the logic should be kept separately and then the Paypal payment triggered on a certain date?