I had a Rhodes 19 on a trailer. Each time I took the boat out it required me to hoist the entire boat (fixed keel) into the water and step the mast. Two of us were able to do it without any problems. So, unstepping and re-stepping the mast shouldn't be that much of a problem. But, it was much easier to handle the mast with the boat on a trailer.
I can sympathize with you. On my current boat, J/36, I got the main-halyard stuck while trying to install a new one. Being a 50' spar and fractional rig, the safest thing was to unstep the mast. As it turns out, the sheave was too narrow which allowed the main-halyard to jump the sheave and get stuck.
Although entertaining, none of this really helps you. I wouldn't try the ladder idea unless the boat was stable and secure out of the water. The easiest idea seems to be to find a structure that's tall enough, close enough to water 3-4 feet deep, that you can climb and be able to use a boat hook or other long pole to "grab" the halyard. Or, if you have or can borrow a trailer, could you safely move the boat with mast upright close enough to a building that would be tall enough with access at a level that you could use a boat hook? The second option might be to borrow a bosun's chair and send someone up the jib-halyard with a boat hook. The least desirable would probably be having someone free-climb the mast.
If you have friends in the fire department or know anyone with access to any kind of crane...now is the time to call in the favors.
Don't forget to calculate tidal heights into the equation.
My sympathy, and I wish you the best of luck.
PS If you're anywhere near Vallejo California, I've got the spot you need about 300' from my boat.