You may be able to get it to work on other systems if you copy / port some of the necessary Apple bits. You'd have to copy the iOS SDKs from Xcode and get ld64 [0] / codesign [1] (both open source) to build on non-Mac machines. Perhaps some resource file prep tools as well. Then you could use libimobiledevice [2] and friends to push apps to the phone and debug them.
Theos [0] has been used with Linux before but I have never personally done so. I don't think anybody has yet ported ld64/codesign to run natively on Linux but it has been possible for some time to run the Mac CLI dev tools under Linux using runtime shims like maloader [1] or its descendant Darling [2].
maloader and Darling won't, they require Linux. LLVM works fine on Windows. I'm guessing the RoboVM front-end is also fairly portable. The effort to port ld64/codesign to Windows would probably be similar to the effort to port them to Linux.
For now you'll need Xcode to use RoboVM which implies Mac OS X running on Mac hardware or in a VM (it can be done although Apple doesn't allow it unless you run the VM on Mac hardware).
[0]: http://www.opensource.apple.com/source/ld64/ld64-136/ (unfortunately the last source release is from Xcode 4.6, 2 releases behind current)
[1]: http://www.opensource.apple.com/source/security_systemkeycha...
[2]: http://www.libimobiledevice.org/