So, the IBJJF has banned belt-grip Sumi Gaeshi (Hikkomi Gaeshi, if you're a pedant) because it "endangers the neck" despite it being a very common technique in Judo and neck injuries from it being basically unheard of. Jumping a guillotine and pulling the opponent face first into the mat with it is perfectly safe though and so is completely legal. 
I've been thinking and it seems the answer to this is to lock around the head/neck and flip them over with Sumi Gaeshi just like if you had a belt grip. Now this is of course many times more dangerous to the neck than belt-grip Sumi and absolutely WILL smash your opponent's head into the mat, but is still legal under IBJJF rules (would get you an instant DQ in Judo, funnily enough).

I've been thinking and it seems the answer to this is to lock around the head/neck and flip them over with Sumi Gaeshi just like if you had a belt grip. Now this is of course many times more dangerous to the neck than belt-grip Sumi and absolutely WILL smash your opponent's head into the mat, but is still legal under IBJJF rules (would get you an instant DQ in Judo, funnily enough).