i have thought about this as well, since i wanted the 2000 year for the MSM color, and the 4wd system of the SR5/highlander, but wanted some of the limited features.....
anyway the problems of this are: The digital has an auto setting, which means it should have a photo cell, as well as inside and possibly outside temp sensors. The other problem is that the manual version uses cables to actuate the duct work, where the digital ones will use servo motors.
Basically it would be cool, but would be a lot of work, and the best way would be w/ access to a full limited model to pull parts from.