This is a bad idea.

There are a couple of reasons. First, you're shitting on their game. You think you're being funny, what you really are is being disruptive. If it was your close friends, I'd say fine, but your first impression is going to be disrupting their game - whether for a laugh or not, I guarantee you that some people aren't going to find it funny.

Secondly, actual burning torches are bad ideas, period. Even the best of intentions doesn't account for accidents.

Thirdly, I hate to say it, but there is some truth to the stereotype that people who can't function well and understand what is and is not appropriate behavior are attracted to RPGs, of which LARP is one. This is why every LARP book clarifies a "no touching!" rule. Overkill? Yes, but alot of larpers don't know the difference between a welcome touch and a bad touch, and don't know that they don't know the difference. You want people to have fun, not be creeped out.

That kind of stunt just makes you look like you're jerks who want to ruin someone else's game.

Here's what I'd do.

Honestly? I'd approach the GM, say "Hey, I heard you were larpers, we thought it would be funny if we dressed up in witch hunter outfits, and came out early on in the evening from the woods, and say: 'Nobody expects the Inquisition!' Then when the laugh is over, we'll get into our regular clothes and start playing the game as normal, or sit on the sidelines and watch as we've never played LARP before. Afterwards, we can all go out and get pizza or something."