Not had these brakes but i would try tuening the brake around on the bars so you cab fill the screw hole with a release agent, Plusgas is still good, but GT85 is better than nowt. Allow it so soak in and do its job. then get a really good/new Allen key with out a worn end and start working the screw slowly.
Take your time it will release in the end
soak with WD type stuff, plus may help if you take it into warm house to help penetration, as much heat as you can give it infact, on the radiator or something.
Top tips – always tighten slightly before trying to undo, it helps break the ‘bond’ on the threadface it is pushing against, and if you can also make this a sharp twist to ‘shock load’ it, as long as this wont **** the screw head