There are several altcoins project choose to add BTC paying signature campaigns. But there's few of them doing this before since maybe there are less good project running around and most of those project posted then choose to pay altcoins are scams.
Indeed its rare to see an exchange running a signature campaign here, maybe the number of newly launched exchange is less compare to those who run a casino that's why its common to see a new casino themed campaigns running in this forum. But for sure there are other project would run a campaign here since bitcointalk is a big platform, for sure project owners would see this as a great platform to market their new projects.