You can use the filter to override the actual link that the title points to. However, it doesn’t help you.
Detecting the url of the current page is quite involved and I am not sure this is something that I want to do in the plugin. You could, however, hack together something in your theme’s functions.php file.
I’d start with trying to get the URL of the current page and then hook into the dynamic_sidebar_params
filter to see if you can find any widgets whose title link is the same as the current url – and then you can pretty easily add a “current-page” (or whatever) class to it.
This is by no means an easy route to take, and requires pretty good understanding on how WordPress and widgets work though.