idk, depends on how its implemented. like an iframe? or can you interact with the imageboard API to get the data and present it nicely here? 4chan, lainchan, wizchan, i think they all share the same base and have JSON APIs you can query.

like maybe you see:
>>>/wiz/1273618273612
when you hover over it, you see the post. or maybe you can click on it to toggle into view.

^ yeah that is a good way to fit in with the site because if you include other sites and just show them always its going to take up a huge amount of the page. this is good advice

^ i think it would be a cool feature. a lot of the time i want to reply to something on tfw or wiz so i just use the quote tag, but being able to directly link to sister site posts would be better.