How to resolve a rusting uPVC window fixing torx screw?
I think I would see if I could remove the rusting screw.  It boils down to, it is never going to be less likely to break off than it is right now.  If you're lucky, you could coat it with silicone, seal out the air, and it won't get any worse, but that's just the head - the shaft may continue to deteriorate.  

I'd try to get it out with just the torque I can work up with a screwdriver with a Torx bit, if I couldn't get it out by hand, I'd leave it.  

I'd also try to get some penetrating oil on it, although it may be tough to get it past the head and down to where it counts.  Also tap down on the head a little with a hammer to vibrate it a little, this often helps break a stuck screw free.