Are there? I'm not aware of any, at least not relating to uhttpd.
LuCI uhttpd is not rewriting any URLs by itself or any directives as far as I am aware. The only redirects are done using very regular "Location" headers initiated by LuCI's (LUA) templates, otherwise there's no fancy internal rewrites going on. Someone CMIIW.
IMHO: uhttpd is very much purpose-built for LuCI, and it does a fine job at that, but deliberately not very much beyond that. If you're using PHP7, presumably to create a bit more elaborate web offerings, you might as well step up to a more capable httpd. lighttpd comes to mind. Apache might be a big ask, depending on the machine you are running all of this on, but then again you're already handling PHP7.
What kind of rewrites do you have in mind? Is it just about hiding the .php extension / hiding the path of the PHP script being executed and simply pass the entire path portion of the URL to the script as PATH_INFO ?