I seem to recall polystyrene breaks down under UV, and can crumble away after a couple of years if it's not covered, so it's probably worth putting boards or paint over it once it's hung. (If you do paint it, again, be careful which paint you use - water-based is generally best.)
I'm possibly over-thinking it, but if you're planning to attach some kind of hard surface to the inside after the insulation anyway, could you not just sandwich the polystyrene in and fasten those mechanically? Attach battens to the wall, stuff the insulation between the battens and then screw boards to the battens over the top, or something? Whatever adhesive you use, the polystyrene will break well before the glue fails - I don't know what the standard practice is, but I wouldn't trust it to hang boards on or anything myself...