For what it is worth, I've successfully straightened and hardened knife blanks using a MAP/Pro torch with firebricks as backing, though I admit to some serious warping issues which were probably unrelated to the heat source. I have no doubt that you could get your trigger guard up to cherry red with one, though juggling the triggerguard, torch, and hammer/pliers is likely to be something of a trial. I've never messed with a cast steel triggerguard, so I can't comment on the workability of the steel at that heat, though.
If you are going to anneal, I suggest you go the coals route, though, as getting the whole piece up to critical heat and and keeping it there long enough to get it buried in ash or sand is likely to be rather difficult. If you have a woodstove and are still running it this late in the year, I've found that throwing in a piece of steel in the woodstove and letting it get to heat before shutting the stove down for the night works very well for annealing old files.
