* check / remove todos
* check if foreign file repo is still needed

from https://gerrit.wikimedia.org/r/#/c/182449/4 Patch Set 2 (RV):
I recommend to use a "dummy" RL module to indicate a "preview" request from the iFrame.
When the 'flexiskin' parameter is set in the requested URL and it is not the 'current active' id, just do something like
 public function onBeforePageDisplay( /* ... */) {
     //...
     $out->addModuleStyles( 'bluespice.flexiskin.dummy.'.$sId );
     //...
 }
This module could then be a custom implementation of class ResourceLoaderModule which includes the requires stylings/resources.
Maybe you could also work with
 ResourceLoaderGetStartupModules
or
 ResourceLoaderRegisterModules
hook.
This could also be a good approach for "normal" flexiskin loading. By this you could avoid using OutPutPage::addHeadItem in the initExt-method