The pistol grip will remove with an ordinary screwdriver, providing it is wide enough. You can make something out of flat plate easy enough as well.
The problem with trying to remove the butt without the proper tool is that there is a bit of a ratcheting/locking mechanism to the inner nut. I was ready to cut one nut off a bren, but I took the time to head into the house, find that takedown tool, and it was off in seconds. I truly thought it was seized.
I do have the Bren wallets for sale on the EE, which contain the takedown tool along with the rest of the goodies.