I tried to overcome this on my GS 430, but failed. What I did was I locate the speed senser wire from the gearbox which supplies the SAT/NAV computer. (Sporties son laying in the boot with a multimeter whilst I drove up and down the road)
I then fitted a timer (Kit from Maplin) to disconect the senser feed for a set time (25seconds) to enable changing of routes etc. Now this did sort of work, however the system in the Lexus also uses the Sats to work out your position.
So when I disconected the speed senser wire the computer would see this and switch to the Sats to workout position, speed etc.
This resulted in most of the times of only periods of 12 seconds before the options greyed out and you were back to square one (as the sats took over). Sometimes it took longer, guess that was how many SAT's the system could see at the time.
I was thinking about disconecting the SAT aerial at the same time as the speed senser to effectively make the device blind.
However I sold the car and now use TOMTOM software on an ipaq, much better than the poor Lexus system.
Hope this helps answer your question ;)