I've used Embedded Ruby (ERB), I was also converting YAML to JSON before CloudFormation added native support for YAML templates (Sep 2016). My company's web application was a Ruby/Rails stack so `.yml.erb` syntax was already quite familiar to our team. Pre-processing the template is a simple shell one-liner that you can easily add to your test/deploy scripts:

    cat template.yml.erb | ruby -rerb -e "puts ERB.new(ARGF.read, nil, '-').result" > template.yml

A lightweight template-preprocessor step adds just enough scripting automation (in a familiar language/environment of your choice) to cut through boilerplate, and avoids imposing yet another domain-specific intermediate abstraction layer on top of the whole stack (e.g., troposphere's Python API, arguably also Terraform's HCL).