I just got me a 4 wheel lifetime alignment at Firestone - and then they told me they can't adjust the rear tires as you have mentioned.
When I discussed this with the Toyota dealer, he said that the rear ones ARE adjustable, but you have to remove the struts to do it, which Firestone would lose their shirts if they had to do that. So now I basically paid for a lifetime 2 wheel alignment
The Toyota dealer did, however, say that for the first year, 12,500 miles, they do a free 4-wheel alignment and adjust/clean the brakes. But it's an unwritten thing. Glad I asked about the alignment! Wish they would have told me this when I bought my 2004 Sienna from them!