Really depends on the type of show. Is it an Arab show, or an open show? A 4-H show? Will there be a variety of breeds?
Speaking from the background of hunter shows, or local schooling shows with a variety of types and breeds, Hunter Hack is usually judged on suitability to be a field hunter, with more emphasis on manners than movement or type. (Hunter Under Saddle has more emphasis on movement and typey-ness, some judges use the Hunter Hack as an opportunity to pin the solid citizens and good do-bes that aren't fancy enough to win in the under saddle.)
Calm, obedient, forward movement with prompt transitions and no resistance are the keys.
If your Arab tends to move with a lot of knee action, or travel with their nose behind the vertical, that will work against him. If he shows with a lot of the fire and animation typical of the breed, that may work against him as well.
Bottom line? If there are 6 perfectly well broken typey TBs in the class with flat knee action that don't make a mistake, you probably won't pin. If there's a mix of horses, types, behaviors and temperments and your Arab looks well broken and pleasant to ride, you probably will.
Go and have fun and let us know how it goes.