Don't know if we count as big yet, but we're doing very very well: https://www.tinfoilsecurity.com
Just tried it on https://beetle.email and got spammed with new signup notifications. You might want to put a warning on there saying that it will submit any forms it can find a LOT. Also, on the waiting screen, while it's running tests, I filled in the form and it's had an iframe drama. https://www.dropbox.com/s/7rofi17cjormgav/Screenshot%202016-...
And thanks for the info on the iFrame - I'll look into that as well.