I'm intrigued. What kind of app that you could host on heroku requires wildcard certificates? Bearing in mind that heroku can't really support wildcard subdomains for a single app. Each custom subdomain for an app needs to be added to the app. And then if you enable Automated Certificate Management for the app (which uses LetsEncrypt under the covers), they'll happily fetch a cert for each listed subdomain.

And Heroku already supports wildcard certs (that you need to provide yourself) if you use the SSL addon.

I have a feeling they've been working on this. Their https support has always been top notch!