Basically, I needed something like this:
Since I've recently played with Trello API, I've decided to make a small script which would help to keep the boards connected, and basically have "column in a task" functionality. In brief, it takes all tasks from the specified column and writes them to the specified task's description.
What it does is grabs data from external board(s) and writes a small summary into the card on the main board:
Yes, it's that simple with https://github.com/gregsdennis/Manatee.Trello!
Here's how it looks on a real board:
The script runs on schedule and keeps the data up to date.
I'm not sharing the entire solution here, as it is rather problem-specific, so let me know in the comments if you need more details.