Monorepo + Pipelines: Can't specify buildpacks for review apps, and app.json can't be in the project root

Question

I'm trying to set up a pipeline for my Monorepo structure similar to this:

 > yarn.lock
 > package.json
 > packages
     > my-heroku-app
         > app.json
         > Procfile
         > package.json
     > my-other-heroku-app
         > app.json
         > Procfile
         > package.json

In the pipeline settings I can set config vars for the review apps which is great, but there isn't an option to set buildpacks. So where other users have been putting their buildpacks in to app.json I can't. In my setup, the app.json file would have to be located in each sub-app (see above).

By the way i'm using this buildpack to handle the separate procfiles, which is what I also need in the review apps. https://elements.heroku.com/buildpacks/heroku/heroku-buildpack-multi-procfile

Is there any work arounds or am I going about this the wrong way. Does anyone have advice for trying to get a monorepo to work with heroku?