Would it be a bad idea to bring a DSLR to a metal concert in a really small venue?
How small is really small?
If the venue is standing room (mosh pit) only don't even consider it. Not worth the risk to damage your gear.
If it's a bit larger with some seating which nobody sits in anyway then there's some other factors you'll have to consider. Lets say you have a seat that is front row, how far from the stage will you be? As it's a night show it's obviously going to be under lights, most likely only stage lighting.
Even the best speedlights (if you have one) have a limited range and become useless after that, and only light up lets say for an example the back of peoples heads that are in front of you for ten or so meters. So you will have up the ISO and slow down your shutter speed (hard to do at a metal concert) to get some good results.
I've been to a crap loads of metal concerts over the last 20 years and can only think of one that I would have been able to get some good shots at. It was outdoors during the day and I had access to an area near the stage that a 200mm lens would have given some great results.
Another thing to consider is are you allowed to take photos at the show? The last show I went to in Melbourne about 6 months ago (Bullet For My Valentine) one of the people I was with had their camera taken of them and was told to pick it up after the show.
So over all think of your gear mate, honestly being in a metal crowd probably isn't the best place that you can look after it. Ultimately it's obviously your call and if you get some some shots please post them up, I'd love to see them.
Cheers Shaun.