Re: What technique would you apply to neutralize Brazilian Jujitsu attacker
I don't really know what I'm talking about here -- very limited BJJ experience, though I'm working on that -- but I thought I'd chip in a few specific ideas. (Please note this disclaimer: grain of salt.)
It's sometimes possible to apply a nikkajo or kotegaeshi to a shoulder grab, if you're wearing a gi. Experienced people seem to know how to avoid this. Also note: you need to do something once you bring them down with that first lock.
I'd recommend keeping your arms out during the standup portion. I notice wrestlers and the like tend to use "hooks", and grab a lot around the head/neck/shoulders. It's kind of hard doing aikido when some guy's pinning your head down with a clinch. If you make sure that they have to go through your arms to get to your body, at least it increases the chances of something happening at more aikido-ish range.
I'm currently thinking that sokumen iriminage might be a doable technique from a katamochi here. Haven't yet tried it; will maybe give it a shot tomorrow.
One issue that occurs to me is the lack of atemi in grappling. I'm wondering if maybe a feint up towards the underside of the jaw or something might function similarly -- something that'll make them flinch or at least divert their attention. (Of course, the addition of real striking might make aikido even more challenging to apply.)