Sorry, I thought you were trying to toggle hiding/showing of Hidden files, as in files that have the H attribute set.
If you're talking about files which are hidden due to Opus's filters, and want a button which ensures that all currently filtered out files are shown again, then have a look near the end of...
...where there's a button to clear all current filters.
You can't make that a toggle button, though, since there's no way to remember the existing filters before clearing them, and thus no way to re-apply them after they are cleared. (Using a button that is. You can always close the lister and open a new one to get your default filtering back. Or make a button which explicitly re-applies the filters, or which specifies a certain Folder Format which re-applies the filters.)
You can toggle specific filters, like .name using commands like HIDEFILTERFILENAME=.name as I guess you've discovered.