Windows RT is basically just Win8 recompiled for ARM, with one major exception: EXE files need to be signed by Microsoft before they will run. This means that MS can release desktop apps just fine - they have the signing keys, after all - but third-party software can't run by itself (as a desktop app) and will need to be bundled as an .APPX file (Metro-style app bundle).
If you want to try bypassing the signature check, there are a few things you could attempt. One would be to create your own signing certificate, install the public key in the OSes root code signing cert store (not the per-user store, though it qprobably wouldn't hurt to install it there too), and then sign your test apps with that cert. MS *probably* used certificate pinning - where a specific cert is used, rather than just any cert present in the OS of sufficient trust level - but they may not have, too. Alternatively, you could try looking for some legacy or debug functionality to disable the code-signing. Finally, you coul try using a built-in program, rundll, to invoke your applications.
I can't test any of this right now, because I don't have an RT device. There's a lot of research on them that I want to do, though.