I am a retired firefighter who was the Fire Marshal for a large department.
I applaud your efforts, but application of water should be the last step in keeping your place safe. First and foremost is that if you are concerned about a grass fire being the ignition source you need to be proactive about keeping a clear space around your structures so the fuel is not close enough to ignite them. Even 20 feet of clear space will greatly lower the chances of fire spread. Look here for more tips:
Wildfire preparedness tips
If you do want to add protection my suggestion would be to use what are known as "deluge" heads. These are open fire sprinkler heads that disperse a chosen water pattern. In your case you can choose a head that deflects the spray pattern down to wet the walls.
When water is sent to the supply pipes all of the heads will flow, kind of like what Hollywood typically pretends all fire sprinklers do. If you use regular heat activated heads the chances are that they will never activate since you have no way to trap the heat needed to open them.
There are many ways to activate the heads, but most are pretty expensive for this type of installation since you cannot use heat as the trigger. Things like ultra violet flame detection exist for situations like this, but they are way too expensive for what fire sprinkler professionals consider to be a low value risk. Frankly I'm stumped about how to open the water supply valve unless you manually do it. The obvious down side to that is you may not be home when protection is needed.
What may be an even better way of protecting your property is to install a fire detection system in the buildings. Smoke detectors will not work, but heat detectors will. You can install most systems for less than $1K and have them monitored for around 25 bucks a month. Plus burglar protection will be included as well. Although remember a fire will take everything a thief only some things.