There is a vendor creating custom tanks for the 200 Land Cruiser on the 200 Series forum of Ih8mud.
Someone with a GX460 inquired about the option for them and the vendor said the 4Runner and GX options are being looked at or developed... It is a separate tank with a separate filling port (standard fill port will be replaced with a port with two holes, so the fill port stays in the factory location.
https://forum.ih8mud.com/threads/auxiliary-fuel-tanks-for-200s.1013044/
If people want this, they should reach out to the vendor to show the demand...