Hello all! I see you are having a very good discussion, and I belive I have the answer. Before I started Aikido I had four years of Karate and som JuJutsu, and quite simply the easiest way to stop a grappler is too....Relax completely! One of Tohei's five ki principles, you see grapplers require resistance, if they were to do say a bear hug and you relax completely and do not resist, there isn't much they can do, and beside, why are you concerning yourself with your attacker? As O'sensei said, attackers will come in all shapes forms and sizes, but they are just energy to be harmonized with.
Relaxing is the base but not all of it. If you just relax with someone shooting at your legs you just go down faster. The key, as in all fights, remain reaction and timing. Someone like BJJer attacks your center and if you don't move fast enough he takes your center. Someone who shoots at your legs doesn't give you the energy like in a punch, push or grab. It is a constant energy charging your center and the only way to harmonize with it would be...well going down with the opponent, thing that Vs a BJJer you wouldn't want.