I like to think that you can use Aikido principles when you are on the ground as well. There are probably a few hidden kaeshi-waza somewhere that lets you spot an opening and take advantage of it. But that might be only if there is an opening. Perhaps if a BJJ practitioner gets you in the crucifix there may not be much that you can do.
There's a video of Shioda sensei on youtube somewhere with 2 of his students pinning him flat on the ground. The next moment he brought them down with some sort of ki technique..
That's what I actually meant.