I've used it for toothpaste for years, in a sprayer (actually an old Dr. Bronner's hand sanitizer sprayer, though I would not recommend that product as it is quite strongly scented). It needs to be very diluted (even a drop of full strength is too much) and tastes disgusting at first but I was mostly used to it after a week or so and now only find it unpleasant if I use more than usual. I'd recommend brushing a second time with just water to prevent aftertaste and since soap in the digestive system can limit absorbtion. The nice thing is that it leaves the toothbrush much cleaner than with regular toothpaste (and is essentially free if you use it for anything else).

I can't really say how well it works; my teeth are in bad shape but that is presumably because I love sugar and vinegar. I think dentists would tend to recommend baking soda as the simple inexpensive toothpaste alternative, however at least one study someone mentioned here not too long ago found that using toothpaste vs water doesn't seem to make much difference. I've heard some theorizing about abrasive vs. non abrasive toothpaste but I haven't looked if there are any studies on that question.

I knew a guy who probably spent most of the year camping and only worked enough to fund himself for the rest of the year. He used Dr. Bronners for EVERYTHING. To bathe, for his hair, brushed his teeth with it, washed his dishes, everything. When camping he carried a small bottle of it and it took care of all his needs.