I would like to be able to type a filename in zsh, hit tab, and see a list of files that match that name in any subdirectories of my current directory. Then I could tab through those results and hit enter to select a certain file, just like the built-in zsh tab completion functionality works.
Example desired functionality:
$ emacs app.css [tab]
*www/css/app.css* tmp/static/app.css
[enter]
$ emacs www/css/app.css
If this is not currently a feature, I assume I could write a custom completion script to implement it.
You can achieve something similar by utilizing the ** pattern (match over multiple directories) and by adding the control function _expand to the completer style (if not already the case).
As a minimum, you need to make the following settings:
autoload -U compinit
zstyle ':completion:*' completer _expand
compinit
Then you can get the desired list with
% emacs **/app.css[TAB]
It even allows for the option to open all matching files at once.