There have been a couple of exploits for bypassing the aforementioned signing requirement. Since all are quite vaguely named, I shall refer to them only by ...
An all-in-on package for Tegra based Windows RT tablets (ASUS VivoTab RT, Lenovo IdeaPad Yoga 11, Microsoft Surface RT & Microsoft Surface 2) Extract · Disabling Secure Boot · Windows Configuration · Jailbreak Killing Updates
I made this guide with my experience on jailbreaking the surface rt, 1 and 2 (i have a lot of them, lol) and i hope this helps someone jailbreaking theirs.
Here goes the instructions for the boot-ready Surface RT 1. Create Recovery usb stick (search for recovery drive in control panel, not the file recovery)