I'm not near a rulebook at the moment, but I do have access to my codex. I like to re-read weapon entries on occasion and see if I've missed something. I just came across this, and didn't know if it's addressed specifically in the codex or not since it's not really something that comes up often as a DE player, and wasn't sure how it would be handled. Again, I don't have my rulebook with me or I would look it up.... so this may just be a brain fart question.
The agoniser entry reads, "Though agonisers come in a variety of forms, the most common are whips or barbed flails. Agonisers are power weapons that always wound on a 4+, regardless of the enemy's Toughness."
So the agoniser can take many forms, is a power weapon, and the only change is specifying what number it rolls to wound. So, can the agoniser be modeled as an axe, or spear, and be used as such? i don't even know if I would want to, b/c I can't remember what those rules were, just if it's even possible.