Using unregistered transliteration modules
It would be possible to add a "module_override" parameter to transliterate in Module:languages. That way you could transliterate texts with a module other than the registered one. Your method would not work, because the language code is also passed to the transliteration module as a parameter.
We should make it clear though that this new function would only be for testing purposes, to avoid people using it too often. Maybe a tracking template should track all uses of this parameter?
That sounds like a good idea. I assume you mean something like {{xlit|module_override=Module:ar-translit}}?
Well, the function I mentioned would also need such a parameter for it to work. But yes, {{xlit}} would also need to support that parameter.
Oh I see, there are more intermediate functions. I think for the transliterate function, "module_override" is a good parameter name, but for the {{xlit}} it should probably be shorter. Something like {{xlit|mod=Module:ar-translit}}, possibly even without the Module: part if we know with 100% certainty that all modules will be in the module namespace.