Add as a first buildpack in the chain. Set PROJECT_PATH environment variable to point to project root. It will be promoted to slug's root, everything else will be erased. Following buildpack (e.g. nodejs) will finish slug compilation.

1. heroku buildpacks:clear if necessary
2. heroku buildpacks:set https://github.com/credspark/subdir-heroku-buildpack
3. heroku buildpacks:add heroku/nodejs or whatever buildpack you need for your application
4. heroku config:set PROJECT_PATH=projects/nodejs/frontend pointing to what you want to be a project root.
5. Deploy your project to Heroku.

The buildpack takes subdirectory you configured, erases everything else, and copies that subdirectory to project root. Then normal Heroku slug compilation proceeds.