This is a proof of concept script for capturing completion matches generated by the zshcompsys completion system. Because of the very complicated nature of zsh's completions, this can (to my knowledge) not be done in a straightforward way, and this script is accordingly hacky.
Roughly, a pseudo-interactive zsh session is spawned using zpty, and a buffer string plus a tab character is sent so the complete-word widget is executed. To capture the hits, the compadd function is selectivly overridden in an inline-sourced file, capturing matches by injecting a parameter to the original compadd call and outputting matches to stdout.
capture.zsh 'vim -' capture.zsh 'vim --' capture.zsh 'vim --r' capture.zsh 'echo *(' capture.zsh 'scp hostname:'
For an application of ths technique, check out vim-zsh-completion.