In case anyone else is looking for a solution for this issue, here is how I solved it.
First, I made sure that
activate :asset_hash was turned on and
set :images_dir, 'assets/images/middleman' was configured for my template assets. Next, I broke up the assets used by webpack and middleman as shown below. This allowed me to use code like this
link rel='shortcut icon' href=(image_path('favicons/favicon.ico')) in my slim templates and have routable assets with hashes to break browser caching.
This looks something like this…