Chrome only: AS2 ExternalInterface calls return null / cross-domain iframes

Question

I'm using the cross-domain hack "marathon version" (explained here: http://softwareas.com/cross-domain-communication-with-iframes) to append a swf to a top-level window from within an iframe, as follows:

1. Page on domain A hosts our javascript, on domain B, in an iframe.
2. Our js on domain B creates a proxy iframe pointing to a file on domain A (call it iframe.html).
3. When its iframe is created, iframe.html on domain A then appends a swf to the parent window DOM, also on domain A.

Works in all browsers. However, in Chrome, any ExternalInterface calls made by the appended swf always return null (the js in the EI calls in hard-coded in the AS2 swf, and is not in any external file). No other browsers return null in this case. Further, if the middle window domain (B) is the same as the parent and its own child, ExternalInterface calls work fine in Chrome.

Is this disabling of javascript/ExternalInterface a security measure unique to Chrome?

Thanks in advance for any help.