@esotericpig from what you mention it sounds like it'd be easy to create a [custom filetype](https://www.geany.org/manual/#custom-filetypes) just adding a few keywords, at least for the highlighting part, if the syntax is basically C++.
For extracting symbols (the [Universal-CTags](/universal-ctags/ctags/) part) it might actually require some extra work if there is different syntax for defining classes or such, as then it's not merely tokenizing but parsing.