So looking at flamethrowers in SWADE:
Flamethrowers propel incendiary liquid or gas to incinerate their targets. They are Heavy Weapons, use the Cone Template (see Area Effect Attacks, page 97), and may be Evaded (page 100). Armor protects normally, but flammable targets may catch fire (see Fire, page 127).
For vehicular flamethrowers, see page 80
I would say take get the armor that the equipment gives if it all give the same boni just from the rules. If the equpiment is different for each part of the body I would use the bonus of the torso.
But as a game master if someone is actually looking for some armor that protects especially well against AOE attacks I would create an armor that offers extra protection against AOE attacks, maybe an additional +2 because it has no real opening.
Or if AOE attacks should be especially deadly take the lowest armor boni if some part of the body is unprotected that means armor 0.