In JS, Cc["@mozilla.org/xre/app-info;1"].getService(Ci.nsIXULRuntime) can be replaced with Services.appinfo. I think for C++, an entry would have to be added to xpcom/build/Services.py for it. One tricky thing is that some tests use updateAppInfo from AppInfo.jsm. I'm not sure how to deal with that. Maybe the C++ implementation of AppInfo will have to be given a method to let you overload values.

For now, this will not remove the XPCOM component registration since Services.foo still rely on it, right?

Oh, right. I guess once things are centralized, maybe the Services thing could be written to load it via some other means.

I don't think it's going to be that easy. In order to replace each XPCOM based service with a C++ singleton getter function, you would need to replace the XPCOM registration code with a singleton getter function on the XPCOM service implementation side, similar to the patches landing for a lot of the blockers for bug 1477576. That is, once Services.foo _can_ use a C++ singleton getter function, then we can start to rip off this band-aid sloooooowly. :-)