Get one of the tiny bottles of soy sauce and take it with you.
Don't be shy about it. They want your business
My sister and I would go to a buffet while dieting that had lots of healthy veggie dishes. I use soy sauce on any rice rather than gravy (and on some vegetables instead of salt). One day they quit stocking soy sauce and offered me worchestershire instead
). From then on I took a little bottle of soy sauce with me when we ate there. There's no reason to let something so simple keep a meal from being enjoyable.
If carrying the bottle in (even in your pocket) bothers you, take a couple of packets in. You can find them in restaurant supply stores or sometimes asian markets will sell bundles of them.