that's an interesting twist to the concept of feature flags, since this is user driven, I can see myself trying it out.
how hard is it to integrate into an existing saas app and are there any requirements for my saas app to be able to use it?
It works on any modern web app. You simply put a script in the header tag of your application and that's pretty much it. After that, most of the "work" is selecting the elements. You can do that either by clicking on any element in the interface or by tagging the element in your code.