Thermal warming is ofcourse a great help, but it is not particularly effective in warming muscles for exercise, this is a fact. Just look at the warm up routines performed by atheletes - you never seen them sitting in the bath before training do you?
I cant explain to you the details of this, but it is recommended to warm the body using mild aerobic exercise - this not only facilitates optimum performance, but also protects against injury. Warm down is equally as important - it is recommended that after training, an athelete should cool down using relaxed static stretches.
Im convinced that these principles would help pianists immensely. Infact, I have been doing this for the last few months and it works perfectly. I take a brisk walk each morning, and this warms my up perfectly for a practice session (my hands movely like greese lightning from the start). To finish I perform some wrist and hand stretches.
The results have been great - no need to spend time preparing for practice with exercises of scales, no injuries, helps with fatigue, has extended my reach significantly.