Safe to say Jive would absolutely not recommend it.
I second this answer. Page (or platform level) CSS/JS has been on my wishlist for quite some time.
If your parent portal is outside of Jive, I expect you're going to see issues iframing your instance as the 'X-Frame-Options' are set to 'same origin'.
Depending what content you're looking to showcase, it's a higher LOE, but you could look at using their APIs to pull content, it won't be the same experience (or as easy as an iframe), but it's something.