Using Hermetic Rune Magic (Major Hermetic Virtue, Ancient Magic p. 136) is it possible to inscribe a rune on a doorway to make it explode when someone enters the room, dealing fire damage to anyone in the room?
No rules here to check, but I can see no reason of why that wouldn't be possible. It can certainly be done with an Hermetic watching ward already.
Rune spells must have the Inscription (Target), the Rune (Duration) and are Touch (Range).
The Inscription (Target) says: "It is the same as Individual (Target), except that the spell requires the caster to write on the target or an object that the target wears or carries."
If they are Individual (Target) spells, they can't be "area" spells, right?
Still, while talking about how rune spells are cast: "The spell affects something other than the caster, or is centered on an object rather than the caster."
Centered? So they can still have an "area of effect" even if they area Individual (Target)?
I don't have AM yet, so this is strictly just FWIW, but for Creo spells, the target is what's created, so a spell that creates a fireball is T: Ind. The fireball thus generated can certainly be big enough to roast anybody nearby.
I think you are right!
LAMP WITHOUT FLAME (R: Touch, D: Conc, T: Ind)
This spell creates a steady light as bright as daylight on a cloudy day. The light has no apparent source, but illuminates an area about ten paces across, centered on a point indicated by the caster. This point may be a mobile item.
I would also add that with a Size modifier you could enlarge the area of the effect.