IE and HTML Bridge with legacyScriptPlugins - "Content blocked"

I'm having an issue testing a legacyScript (i.e. a HTML portlet page running off a server) with IE 11 (and 10).

According to Fiddler web proxy it seems that vCenter UI does a 

CONNECT 10.0.1.147:443 HTTP/1.0
User-Agent: Mozilla/5.0 (Windows NT 6.3; WOW64; Trident/7.0; rv:11.0) like Gecko
Connection: Keep-Alive
Content-Length: 0
Host: 10.0.1.147
Pragma: no-cache

BEFORE the rest of the plugin content is obtained via HTTPS

I'm hypothesizing that this causes IE to see mixed content (secure and insecure content within a single page) and to block the page from being loaded.

This is not the same as getting a sad face if you have a self-signed certificate - IE has already had the server page loaded outside of the iframe to cache the certificate.

I doubt there's much that can be done at this point (except documenting the issue with HTML bridge, legacyScripts and IE) - but maybe next release the client could avoid the CONNECT ?

Any enterprise suitable workarounds for IE or should we only qualify Chrome ?