Fred - you know I disagree with you about the site of the Government Magazine - because 1). the site of the magazine was owned by the Government and these other riverside sites were (and are) all owned by Morden College and 2) because plans of the Magazine show it alongside Tudor Bendish Sluice which could still be seen emerging out of the Enderby site underneath the steps into the river alongside the Enderby jetty - it has just been extinguished and removed by the current builders on the Enderby site (one of the last remains of Tudor Greenwich). 3) neither the plans of the Magazine nor the one picture of it, nor the records of it, mention a ramp.
I think what you say about the ramp is very interesting. And I am writing this from memory so I may be wrong. If you look at the 1880 Ordnance Survey map you will see that a site near where you mention between Pipers Wharf and Granite Wharf is marked as the 'District Board of Works' - the local authority before Greenwich Council was invented. Marked on it and heading towards the river is a ramp - in fact if you go into the same yard now you can see the remains of this ramp inland. My guess - and this is a guess - that the District Board of Works built the ramp for the dust carts so that they could take them to the river and tip the rubbish into barges to be taken off down river. Later they moved this whole operation down river to Tunnel Avenue Depot - and you can see the big new E shaped jetty still there near where the Amylum silos used to be. So your Dad could have been right - if it was owned by the Council it was a 'public' ramp.
Hope you don't mind me saying all this