Using a context free grammar to generate sample utterances for Amazon Alexa/Google Assistant?
I don't think that will be effective.  I don't recommend this approach.  I think there's a significant risk that the machine learning will just learn to handle those specific synthetic variants, but not get any better at handling real speech.  It might even get worse, if you cause the ML to overfit to your synthetic variants.

The problem is that real speech has a lot of variety in how people express things.  Your synthetic examples are not going to capture that variety.  Training machine learning on synthetic examples that aren't representative of what it will see during deployment is not likely to be effective.  I suggest you try to obtain more realistic data, even if it can't be easily programmatically generated.