Creating glass without using translucent blend mode?

depending on the look you’re going for, maybe a custom shader with a bit of parallax could do the trick.
But that would be a different material anyway.
I’m not sure a sixth material would dramatically increase the cost of your character. Worst case scenario, It’s be the cost of a separate glass mesh (skeletal). Since I don’t think there’s any cheap workaround, you’d better go for the simple solution of a new material. And if you’re really worried about the perf, maybe have a closer look at those 5 other materials to see if they can be merged.