It's not as hard as it sounds, but there are a couple of things that you need to be aware of. I found the instructions on the Brother homepage to be somewhat incomplete, but luckily I found clues enough here and there to piece it all together.
In the install I have used the filenames for my printer model, these you should of course replace with your model.
To be able to install the drivers we need cups (obviously) and rpm to be installed. If you don't have them go ahead and emerge them now. Lucky for us rpm is in portage.
Now download the rpm files for your printer from the brother homepage. You need both the lpr and the cupswrapper package.
First install the lpr package. (the order of installation is important)
If it complains about a missing directory, just create the directory and run the command again. You may have to force rpm to reinstall the package by using the option "--force"
Please also make sure the created directory has the correct ownership and permissions.
Next install the cupswrapper package.
The following steps was necessary to create the brlpdwrapperdcpj925dw file in /usr/libexec/cups/filter
[root@workstation]# ./cupswrapperdcpj925dw -i
Then you need to make a symbolic link for cups to be able to use the driver
To be able to install the printer you have to restart the cups daemon.
Then just add the printer using the cups interface (localhost:631). The only thing you should be aware of, is that the address cups detects for the printer (wins name I guess) will not work. You need to specify the printer as "LPD/LPR Host or Printer" and enter the IP address instead.
Also note that by running the cupswrapperdcpj925dw script a printer is generated in cups, but it is set to use the USB connection. If you don't need this one, either change the connection on it to suit you or delete it.
Some pages that may be of interrest
I'd like to commend Brother on their support of Linux. It's nice to see some drivers that actually work for a change. Not all manufacturers can supply that.