Theoretical suggestion:
Wouldn't it be possible to set the "root alias" for the different languages in Mle CMS to "redirecting links" sending visitors to the correct domain?
All domains should off course point to the same cmsms install. (Just realised this could create a loop

Make sure to use the "redirecting links" to send users to the main pages for the languages, not to the "redirect" page!)
I'm not certain about the links created by the wysiwyg and in the menu... If they are "relative" you should be fine, if they are "absolute" they will redirect the visitor back to the main domain so you should look in to that.
You will also need to customize the "Multilang template" and generate your own URL's to allow users to switch languages (and domains).
Now, the $lang_href value does it all for you.
Every record in the "foreach from=$langs item=lang name=language" in the "Multilang template" contains the following information:
->0 = Array (5)
id => "1"
name => "Nederlands"
locale => "nl_NL"
flag => "images/MleCMS/Belgium-32.png"
alias => "welkom"
You will need to write something like this instead of using the regular $lang_href:
IF $lang_locale = "nl_NL"
domain1.com/$alias
ELSEIF $lang_locale = "fr_FR"
domain2.com/$alias
ps: you will have to make sure you Google doesn't penalize you for duplicate content however! Maybe setting up cannonnical links would be enough, maybe not... Just warning you...
Let me know if it works out for you!
Greetings,
Manuel