First of all, I'm very happy to hear that Hanami team is considering Webpack. Webpack is an awesome tool to bundle/process assets.
I've created the hanami-webpack, a very experimental and hacky gem to run Webpack with Hanami, but it's important to tell that I just followed some principles that I saw in https://github.com/mipearson/webpack-rails.
The answers to your questions:
In my gem I used the Webpack's
StatsPlugin to expose, through a JSON manifest, the path for my webpack entry points, so the Hanami can download this manifest in order to figure out the paths for the entries.
Here some key points about the implementation:
About the server, I totally agree with you about the smooth integration between webpack dev server and Hanami, in my gem I made a monkey patch on the Hanami::DevServer.start to spawn a process with the webpack dev server https://github.com/samuelsimoes/hanami-webpack/blob/master/lib/hanami_webpack/dev_server.rb#L7 to make the dev server integration transparent, I don't know if it's a good idea, just sounded good for me.
I'm not a specialist in Webpack or Hanami, but I'm glad to help you guys on this topic.