Hello,
Extension-less URLs shouldn't be any problem even on Cassini. Please check what you have set for the
Page not found URL option in
Site Manager -> Settings -> Your_Site -> URLs and SEO. I tried it on my site and if the extension-less URLs are set in the web.config file (as described in that article I sent previously), I'm correctly redirected to a chosen page after 404 response.
Best regards
Ondrej Vasil