Lexus Woodford fitted a detachable towbar to my new RX400H last July without question.
The towbar is made by Westphalia-Automotive, type 335305600001, max nose weight 80Kg, Towing capacity 2000Kg
Lexus will probably charge £800-£1000 to supply and fit this towbar.
You can save a heap of money by sourcing it on the internet and fitting it yourself (that's what I should have done, since the installation is not difficult)
Towbar details (including fitting instructions) at: http://www.westfalia-automotive.de/index.php?id=54&L=1
A very small cut has to be made at the bottom of the rear bumper, but you wont see it unless you bend down
Hope this helps
BMcP