I did find a “home made” device that is exactly what people need and sold by a company called Aivaka (just Google Aivaka).
They manufacture a perfect solution to this problem, their site have a Product page and an FAQ page that answers most questions.
Their WB100 product basically creates a wireless connection between any Controller output (24VAC) and any 24VAC Sprinkler valve (or 6- 24VDC with their WB200)
Basically they have a transmitter that you connect into your controller valve output, and a receiver that you connect to your valve. The receiver is powered by a small solar panel and a rechargeable battery.
The only problem is that they know there is no solution out there that actually solves this problem, and they mostly sell to big companies so their price is beyond reason for a homeowner (Almost 4x the price of the rachio 3…) 
@franz @rachiosupport you should look into developing a similar solution, this would increase the functionality of your system! 
One other possible source I need to investigate is Tucor’s Nexus Wireless Valve system, but I cant find much details about it.