Since you can already gain the bandits trust and enter the camp through the front door, I'm guessing that you are wanting to have a bit more to go along with that...
Here's an idea:
The Flaming Fist officer who halts the party on the way south from Beregost to Nashkell keep him from appearing until after the player has joined the bandit group. The player is then given a task to go intercept a caravan of iron going from Nashkell to Beregost. Since the iron weakness has been stopped at the mines, the bandits will want to keep the good iron from getting out to the public. When the player returns to that area, the officer will appear and have his normal dialog. Should the player choose to turn themselves in they could be escorted to Officer Vai at the Jovial Juggler. Here the player can explain what they have done and either make coordinated efforts with the Flaming Fist or go it alone and bring back all those bandit scalps as proof...
I'm sure that there are more details that need to be worked out, but you can get the picture...
However, without making a new map you can't have Tazok's tent burn. You can, however, work in something where the bandits have moved Tazok's stuff to a safer location. Perhaps you can catch up to them on their way to the Cloakwood mines....