How funny, I had exactly the same conundrum yesterday when moving the boat (Bav 40) to fill her up on the fuel pontoon single handed. The wind did drop so I was able to just line off, and hop on before she blew onto my neighbor.
I use the midships cleat and line a whole lot, so my method was going to be to shorten the midships spring so it was around the pontoon cleat and then back to the boat, with the bitter end on the boat. Start chugging slowly ahead, turning the bow away (in my case to port) and make sure both bow and stern lines were slack.
I could then undo those at my leisure with the boat "sucked" onto the pontoon by the midships line and engine. Get onto the boat and shut the gearbox into neutral (the boat will start to drift a little), step calmly forward (looking like you know exactly what you are doing), untie the bitter end of the midships line and pull that through. Trying not to look hurried..., run like stink back to the cockpit, trip over, recover, swear a bit, and then give it a really good stab of astern (remembering to centralise the rudder) and then throttle back. That'll get the wash quickly over the rudder, as the saildrive is so much further forward than the rudder, if you go gently, you won't get steerage until too late.
Then hope and pray no one was watching and that your insurance will cover the damage....