I have more than few clients that would like to add facebook connect to their landing pages (managed by me). They are too many and not enough tech-savvy to manually create a
Apps are not meant be used on several different domains, you will have to create a new App for each domain, i´m afraid. You can use the different platforms in the App settings to use different domains, but there are only a few so it´s pointless. Just create some screenshots and a tutorial for your clients, that´s how it is usually done.
Btw, it would be weird to authorize an App on a website, and the same App would allow you to be authorized on all other client websites. Also, insights are per App, so your clients may want to see their own insights and not the global insights of all domains together.