Spray it off at the coin-op, drive it home and do an ONR there. You can surely get water SOMEWHERE, even if you have to buy a few gallons of drinking water at the grocery store...
Sad thing is, I've actually done this before. I had just detailed my car and I was traveling to FL and some idiot in front of my decides to toss his drink out of the car. It must have been full because when it hit the pavement it busted open and just sprayed a cola type drink all over the front of my car. I had a mason jar of ONR in my trunk because I was going to give it to a friend to try in FL, but I just couldn't stand not doing something about it right away. I couldn't find any place that had the coin-op style wash, and I wasn't in a familiar area, but thankfully, I found a super wally's right off the freeway. Grabbed a bucket and a couple gallons of distilled water at $.35 a piece, and ONR'd my car right in the back of the parking lot.
In the end, I felt a little silly, but it was worth it because I just knew that in the summer heat that crap could have caused a ton of damage.
Ever since, I keep an emergency bottle of quick detailer and a large zip lock full of MF's in the trunk.
One more thing....what do I do about the undercarriage since there's no coin-op around? Do I even really need to worry about it that much or do I need to find a way to clean it?