stop mixing terminology. Windows 8.1 which is a software distributed OS has very little to do with WindowsRT, which is the Microsoft iPad.
WindowsRT does not support drivers that are not Microsoft-signed. Windows 8.1 in contrast will happily run all sorts of dubiously signed drivers, unless the driver is specifically blacklisted (Either for being malicious, eg rootkit drivers, or for being unstable on the newer OS, eg the WinPcap driver on Windows 8 when Win8 shipped)
There are literally thousands of Ethernet drivers for Windows 8.1, because most every driver from Vista upwards just work on 8.1 out of the box.
Windows 8.1 blocking drivers has nothing to do with the driver signing requirements of WindowsRT. They are entirely different concepts designed by entirely different teams in entirely different top level departments at Microsoft.