Is it okay to load another page in an iframe in a modal pop-up window - in terms of legal and ethical standards around displaying 3rd party content?

I remember a few years ago there was controversy and a debate about whether it was okay to load another domain's page content on your domain in a full-width iframe, with your site providing a masthead with controls for favouriting, linking etc (e.g. like StumbleUpon).

I seem to recall that the consensus was, that it was okay so long as you were clearly in no way claiming ownership of the 3rd party content or attempting to modify the content and so long as there was a 'go to site' button or equivalent; and that sites could ask you to exclude them, but generally speaking, it's an acceptable practice.

How acceptable would it be considered to be to load another site's page within a modal (lightbox-like) popup box (following all the above principles: clear attribution and a prominent button that kills the iframe and gives them the 3rd party original)? My expectation would be that it would follow the same principles, and be acceptable so long as these conditions were met.

Note that I'm asking about the likely legitimate responses of the 3rd party sites and possible legal position, not about usability or UX. I'm aware that this should never ever ever ever ever be the standard way external links are loaded, and that 99% of the time linking to external content like this would be terrible for usability.

My specific use case is one of those 1% of cases where loading a separate page in this tab actually wouldn't be the expected behaviour of a link: an interactive data visualisation tool that also acts as a 'browser' of external content (science papers underlying the data it navigates). All other links within the interactive will change something while staying on the same page. If the user clicked one of these external links by mistake (as people often do, even when they are clearly, noisily labelled) and then had to back-button back, they would lose their fine-grained position in the interactive tool (jquery bbq hashchanges being not appropriate for all elements of the tool). New window/tab will simply open the target page on the 3rd party domain.

Opening a new window/tab would also be an alternative option (and has its own disadvantages) - my question is, whether this is an alternative that could be considered (in terms of acceptable practice around intellectual property etc), irrespective of which option is best for UX: which is something we'll decide the proper way, based on actual UX testing.