Normally, doing a plugin to implement a sponsor API is doable (not sure there are even one exception, as most of them are just javascript), it is just not the "normal use of C2" so most people just do not want to do it, or just do not know (as it requires javascript knowledge).
Heck, maybe even the Execute javascript of the browser object could be used for that. Is it worth the time however, depends.. if you have to implement it for each publisher, it could be bothersome for some.