Trigger external site launch from within Marketo

We're developing an app that provides content for emails created in marketo. The problem we're having is that this content requires curation actions from the user so the existing webhook structure which is just a data transfer api is not suitable.

What we need is a way to launch and external site (as an iframe or pop-up window) from within marketo and pass it certain context information. Eloqua uses iframes for this, but I haven't found anything similar in marketo.

Thanks.