Seems like it is not possible to disable after all...
I've also been waiting for the dealer to get the software upgrade afer being told by them that it was possible. I called Lexus GB customer service this week and got the following information back on the warning beep and why details of disabling it are in the UK owners manual:
"I have done further investigation into this issue with our technical
department, and they have indicated that the RX300 does have an adjustment
for the beeping signal of the RX300 tailgate. However, the sound volume is
automtically defaulted to the lowest set volume. If you are, however,
referring to the sound from the interior cabin that can easily be
de-activated at your Lexus centre.
Unfortunately, the beeping sound when the RX300 tailgate is closing cannot
be de-activated as this is standard feature on all U.K. specification
models. It has been decided to be a safety feature that is required for
this region, U.K. models are thus assembled as such so that it is incapable
of de-activating. However, other foreign models do allow for this feature
to be de-activated.
It follows, that we use a standard set of manuals that incorporate all
models world-wide, and this means that all features are included for every
RX model. However, some features are region specific based on the safety
requirements and the like.
I hope this clarifies the issue, but please be assured that our technical
department are looking into this issue with our management team and will
look to include this note in future manuals."
Oh well, would have been nice though.