I found the problem: there is a security restriction disabling XSS attacks.
Then, I won't get to process returned data using iframe.
I tried also to use HttpClient or HtmlUnit to process internally the captcha,
save the image in the server HD, and to call the destination form with value
+ captcha (typed in a field manually), without success.
is there any specific reason why to use Captcha from an remote server?
Couldn't you just generate it on your server?
I can imagine Seam's Captcha  will work pretty nice with <a4j:mediaOutput /> .
The problem is... the other site with captcha is not mine, it is a government site, that allows public search of enterprises data (like address, etc), but there is not something like a webservice, to allow programs like mine to search these data automatically.
You can use PhantomJSDriver as a close to real implementation of headless browser.
The similar issue is addressed here - that guy is trying to use PhantomJSDRiver from servlet.