There is definately no reason not to do it.  My job is designing sprinkler systems for commercial and residential buildings.  The benefit of sprinklers in your home can be priceless as you have already guessed.

For what you are wanting to do, it would be very simple.  Since you are not required to have it, you don't have to follow any of the NFPA13 codes.  Just tap the closest water line, install a small globe valve so you can shut it down if need be, and pipe to where you want the sprinkler head.

We supply local plumbers with a few heads all the time around here for similar applications.  Sometimes people don't want the cost of an entire sprinkler system but do want some "special" room of theirs protected, just incase.  Therefore, the plumbers can just put 'em in.

Just make sure you put a valve on it and you should be good.  If you have any questions about anything sprinklers feel free to IM or email me.

Jeff

I understand the concern for not wanting the water in the line for the head getting back in the pottable water system.  You could use a check valve for that purpose.  Or, no dead areas like mentioned before by not trapping any pipe and having it all pitched back toward the point of connection for complete drainage.

You will definately want a valve at point of connection so you can perform maintenance if need be.  And, you could put a tee in after the valve, on the sprinkler side, and plug the opening.  The tee would be your drain connection for flushing.  This would be my preferred configuration.

In normal sprinkler systems using black steel pipe, the water that sits in those systems gets to be extremely foul black soup.  For your system though, you're probably going to use small (1/2" maybe) copper pipe or pvc, so you shouldn't have to flush that pipe very often at all.