This is foolhardy IMHO.
I have put 135,000 miles on my ES300 which is similar in weight distribution, drivetrain, etc. I've also put 55,000 miles on our '04 Prius. I have NEVER had the front tires wear out before the back tires, they have always worn out all at the same time. If you think about it you never give them a chance to if you rotate them every 5,000 miles as per Lexus.
In fact I have done this with every other car I've ever had, Lexus or not and the result has been the same...all the tires wear down at the same time...never the fronts before the back or the back before the front.
I think the best thing to do is to do what Lexus says, rotate the tires every 5,000 miles and if your experience mirrors mine they'll all be ready for replacement at the same time. Anyways you're better off having 4 tires with similar wear and treadpatterns anyways than trying to replace two.