Great idea. Two potential problems:
1. The "Rain Sensor" may not be a stand-alone device -- it may just be another input to the Body Computer (BCM) in the donor vehicle, and the BCM actually controls the wipers. Interfacing a "foreign" BCM is a big undertaking.
2. Judging by what Toyota gets for a "Blower Amp" for a Sequoia (hint as to one of my upcoming mods) -- over $100 -- I would suspect the "Rain Sensor" to be mucho expensive. Although, I think the device is based on something like this
Infrared Range Detector manufactured by Sharp, so it may be possible to build your own system. If I get time, I'll look into it further.