How to add custom permalinks / slugs for pages

My team is building a large-scale marketing website with many infrequently updated pages. These pages should have URL’s like /calculators or /kb. But if we save these in the root of the source directory there will soon be hundreds of them. I’d like to save them in a series of folders without that folder structure being reflected in the URL.

I remember how to do this from back when I wrote Jekyll websites: just include a permalink key in the YAML front matter for a given page. But I don’t think this works with stock Middleman. Additionally, I’d rather not install middleman-blog unnecessarily since these aren’t blog pages.

Is there a feature I’ve missed in the documentation? Otherwise, can someone direct me to an extension that adds just this feature? I’m happy to write my own extension and open source it too. But Ruby isn’t my strong suit and I’d like to avoid duplication of efforts wherever possible.