With a log lighter, that is one of my main concerns. I simply do not understand why more people have not been injured over the years by these things. However, ideally, log lighters have two valves:
one to throttle the flame and a shutoff upstream. Hold your breath on that one...

The OP, however, discussed a capped gas line so even if the shutoff was left open, it won't go anywhere. I know of no incidents involving a capped gas stub-in relative to a gas fire or explosion. There are cases of fire extension through the unsealed conduit into the chase. There is the possibiliy of ignition inside the chase from heat conduction down the iron pipe as several mfr.s have stated clearances for this pipe as it exits the fireplace outer wrap. It would take a Level III or fire to access this area but it is a concern.
Bob

__________________
Keep the fire inside the fireplace!
IAFCI Regional Director & Certified Fireplace Inspector
Certified Master Hearth Professional & HPBA BoD