Hi!
Any update on this?
I'm just now testing out polylang as a replacement for WPML which in my opinion has grown into a monster performancewise..
So far the only thing I feel I'm really missing is this feature.
Perhaps you could look at how WPML does it, I'm no expert on domains and DNS etc. but to me it seems that all WPML really does is let you input a domain for each language which it will then redirect all of that languages urls to use. I'm sure there's more to it since it requires some sort of htaccess (I'm guessing) but to a better person this might not be an issue?