*sigh* so much misinformation. Let me go through this step by step.
For one, Windows RT is locked down... you cannot install apps compiled for Windows RT onto a Surface 2 unless those apps are in the app store.
The procedure for sideloading apps is the same for Windows ARM and Windows x86.
1) get a sideloading product key (or optionally developer unlock your device and skip 2)
2) activate the key with Powershell (slmgr.exe)
3) compile an appx package either for "anyCPU" or just "ARM" (note there is no compilation option for Windows RT, just ARM, x86, x64 or anyCPU)
4) deploy the app package using Powershell (Add-AppxPackage)
Only MS Office RT and the remnants of system Tools
I would not call them remnants, as there are much more system tools, which you apparently are not aware of, as for instance the system tools for sideloading i mentioned above or tools to manage and manipulate ACLs etc.
A second one, Microsoft has not opened up legacy APIs that would utilize the desktop for Windows RT
The legacy APIs are just available. They are not closed/locked at all. I have compiled quite a few desktop apps for ARM using Win32 API.
You can even access legacy APIs from modern apps if you want to. (of course your app will be not allowed in the store if you do so), But that the APIs are locked is a myth.
The only real restriction is the certificate enforcement on Windows RT, which prevents any homebrew desktop app from actually running without jailbreak.