I've created custom templates to do some additional code generation, and I need to not include some relations in the generated code. I can identify these relations because they all have a standard prefix in their name, but there doesn't seem to be anything I can do with this in TDL. Is there a substring function I'm missing that I could use with StringValueEquals? Or some other way I can identify the relations I don't want to generate?

You could opt for:

A. Write a custom property on those entities that wont be included, then identify these entities by their custom property on the template.

or

B. Use LPT templates to gain flexibility on your templates.