1993 M900- It begins.....

[utopia]

Its a bit extreme, but Mr Gremlin once told me that concentrated nitric acid will dissolve steel but leave aluminium untouched.
But I can't confirm it from my own experience.

But it'll be preferential galvanic corrosion of the zinc plating, forming some sort of oxide, which is bunging up the works, I reckon.
So if you can dissolve or soften that with the vinegar then you might be getting somewhere.
Maybe even vinegar and heat.

Or.....I just caught myself wondering.......
Could you put a small location hole in the end of the bolt (using a centredrill perhaps) and then, using a pointed thing in the chuck of your electric drill, which was set to hammer, apply some high frequency hammering to the end of the bolt to jar the corrosion loose ?
Of course the pointy thing would just spin in the hole, but it would still apply the high frequency hammer blows.
Just a thought.
You could file a point on any old bit of steel rod that would fit in the drill chuck.. Even a very fat nail might do the trick, though something beefier might be better.