Aristeia

Ok, so you're sparring with a BJJ'er and he's getting you in mount, then submitting you and you want to know how to escape? Hmm, if only there was someone nearby that is used to being in these positions and might know how to counter them. IOW, why don't you just ask the guy you're sparring with? My experience has been most MAists are happy to share their knowledge when asked.

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.