First of all thank you very much for the clear answer.
In the meantime i've done a lot of testing to get this working with jQuery, and eventually managed to get it working through jsonp.
Although jsonp can't be used to do a POST to a controller, a GET with the json as a parameter worked like a charm.
CORS can be enabled even through the web.config by adding the appropriate custom headers.
The only problem left is browser incompatibility to read request headers through jQuery.
So that makes it nearly impossible to maintain session state on the server, which i needed for a Captcha check.
Eventually i solved the problem by letting my customers embed our web-form through an iframe so i didn't have to worry about CORS.
The project i'm working on is at the moment still an MVC 3 project, but i will probably upgrade it to MVC 4 and use breeze.js (since the asp.net team abbandoned upshot.js and are a bit slow with alternatives).
Thanks again.