The best way to drive the backstay is a belowdecks attachment to the bottom of the chainplate, if that is accessible. No big worries from corrosion
Why? Because the "antenna" begins at the drive point of the tuner, not where a wire might attach. If you run a wire parallel to the chainplate and backstay much radiated energy is lost
into the parallel metal. At or near a quarterwave frequency the bottom of the backstay has minimum voltage and maximum current
creates magnetic field. That is the field that you do not want to lose energy from in order to have the best effective radiated power.
Now for those who are concerned about "shock"; You cannot get shocked by HF you can only get burned and even then you can grab the backstay with max continuous power out (not what you get with SSB
modulation, I'm just pointing out the worst case scenario) and not feel a thing. I have done this with 10kW output continuous. If you brush up against the stay at the top of the antenna you can bet a whitish RF burn on your skin. Not likely at the bottom. So, it is not necessary to run a "High voltage" wire from below decks and attach it high up where theoretically someone might touch it.
Most people including so-called experts do not even seem to know the physics about this type of attempt to get a quarter wave resonant drive element comprising the backstay.