I have thought about - but haven't done it, there being so many, many other things above this particular project on The List - taking a suitable soldering iron, getting it to full heat, and applying it to the screw for a while, then letting it cool, and seeing if that had freed it up.
Were I to get them loose, I'd be tempted to find setscrews with suitable tips and Allen (female hex) openings on the end. Not historically correct, but easier to adjust in future.
A friend gave me a reprint of Stanley's Catalog No. 110, which describes these, and explains that adjusting them for level/plumb involves loosening the setscrew on the low side, inserting paper between the body of the level and the vial, and tightening the setscrew. A touch inelegant.