I wonder how one could achieve that, personally i could pass another vertex data first would be actual geometric position, second would be next vertex in array. But its way too overcomplicated, ill have to build two sets of arrays so i just don't.

Can't actually think of something. Something that would not force me to pass another attribute to shaders, something that wont force me to change my internal model atructure at all,

By the way im drawing lines with usage of GL_LINE_LOOP

Any thoughts ?

If you don't wan't extra data a simple checkerboard pattern is the only thing i can think of to guarantee dashes at every angle.

Found this paper: https://hal.inria.fr/hal-00907326/file/paper.pdf