Can I make my own kernel extensions on El Capitan?
Rootless can be completely disabled on OS X El Capitan (though not on iOS, for hopefully obvious reasons). Therefore if you don't mind foregoing the ostensible benefits it provides, you can essentially revert to Yosemite's security model - which is to say, unsigned kexts can be used, but it requires preparation to do so.

It may not be able to be disabled in future versions of the OS, but for now it can be turned off.