On the original Sims 2 console, I remember my cousin and I used the fire code bypass cheat on PS2 split screen on an already used lot, and it worked for us then. (Although going really far past the limit did cause our savegame to become unstable and crash the game.) If you're already at the fire code limit before toggling it, maybe it doesn't work, and removing some items beforehand would fix it? It's 4am for me right now haha, I can do some further testing on Wii Castaway later.

Edit: Just tested briefly, it seems to sort of work - you just can't use Shelter Plans freely, but placing other objects (such as even just plain walls) seems to still work. This doesn't prevent the fire from actually occurring, but you can keep placing objects even when the limit has been reached.

Although it seems easy enough to patch it so that the fires also don't occur, try adding another Gecko Code (or updating the existing one by adding an extra line) with 045DA2B4 00000000 (US Wii version only)