There are way to many pages for me to get through (I did try) so I will just add to the end, and people can refer me back
If you look at all the "(martial art of choice) vs. BJJ" on youtube, there is one thing that the Gracie's (which are in the majority of them) have that the other martial artist do not have. That is spirit. To be effective in a martial encounter with Aikido you must have the superior spirit. All of those guys on youtube have no chance because their spirit is nothing compared to that of their opponent.
The important thing with Aikido is you must "defeat" your opponent at the moment of contact. With mindful, consistent, and hard training, specifically randori, you develop awareness. Your awareness and the basic principles of Aikido, such as protect your centre (line), you will be aware of the attack, and you can neutralise it at the moment of contact.
If you look at these youtube videos you see the people spar as they train. The other martial artists are not used to the rushed clash and frenzy of the fight. The BJJ guys train this so they can handle it.
You fight how you train!
The wonderful thing about someone who does a grappling art is they want to grab you. So in theory GOOD Aikido should be good to defend against someone who wants to grapple you. The fact is ALL the early greats (Mochizuki, Shioda, Tomiki, etc) were judoka, and old school judoka, so their ground fighting was identical to BJJ (for those who don't believe this, look at material from Mifune and Kawaishi). All of these men were students of O'Sensei because they could not beat him (The story of Shioda comes to mind). They were proficient grapplers and could not take down O'Sensei. So GOOD Aikido will enable you to defend against a BJJer.
As for techniques, according to the source (O'Sensei), "In combat Aikido is 90% atemi." So at the moment of contact a properly placed atemi should be sufficient. This may sound naive, but I am not talking about just trying to hit the person, remember I said you must have mindful consistent hard training to develop awareness. With superior awareness, you can defeat any opponent. You remember all those stories about O'Sensei and his spider sense, awareness! His ability to use "simple" techniques to defeat all opponents, including swordsmen, awareness! If you are aware of how your opponent will attack you, you can do anything you want to defeat him. A BJJ is someone who won't easily tire, and will keep coming, so it has to be lights out. A solid strike to the side of the chin is the proven way to knock someone out with atemi. The rapid turn of the head as a result of the blow, causes the vertebrae in the neck to separate slightly. The result of this is an emergency shut down of the brain to make sure damage does not occur (the scene from Snatch with Brad Pit and the big bare knuckle boxer at the caravan park comes to mind).
Alternatively you could do what we do in Yoseikan, and train in grappling. Personal, I like the fact that O'Sensei didn't have to do this, so this is what I want to achieve.