Or are you looking for a specifically Aikido response to the takedown/mount/submission? If so then I have some bad news for you. There ain't one. Which of course isn't to say there isn't an AIKI response. If you take the concepts that gave birth to aikido and examine how they may apply when your feet aren't on the ground and you have to find completely different models for movement, and then practice that application throughly testing it against an opponant, you will find many ways to counter. This is called BJJ.
I agree and disagree with you. I agree that you could come to BJJ with that process, but I also believe that the way Brazillian Jujutsu is taught in mixed martial arts is not very aiki-friendly. Professional Jujutsu stuff can be very fluid though.