The way that "signature window", really "preview window", works is this:
:help 'omnifunc', if there is one, returns a list of dictionaries, one for each completion suggestion.
- Those dictionaries can have a non-mandatory
info field that contains the text to display in the preview window if
preview is in
- The preview window is triggered automatically when
preview AND when the info field is populated.
In this case, that
info field is populated on the fly by
ccomplete#Complete(), with info gathered from tags, files, etc.. It is the value of that field that is displayed in the preview window and there is currently no built-in way to get that exact text in a preview window or elsewhere without using omni-completion.
Now, all the code used to generate that
info field is under
$VIMRUNTIME/autoload/ccomplete.vim (and it even has been rewritten in vim9script) so it should be doable to cobble something up out of it.