Father made (in my humble opinion some good suggestions) but the thing I'd echo on this one is DOUBLE CHECK WITH INSTRUCTOR. Every school is a bit different about doing what I consider "borderline" moves...moves where you are going to inflict a bit of pain and discomfort on an individual. Then it matters from individual to individual as well...some are ok with letting your crossface the crap out of them or using some of the techniques Fatherdog suggested.
To me there is a lot of gray area here...I hate when people knuckle the **** out of your jaw trying to get you to move your chin to get to your neck...and I swear my jaw was about to break at times...but I'd feel like a ***** if I gave it up and am willing to deal with a sore jaw for a few days. Other people may feel this is stupid on my part and may not want to deal with the same...I don't look down on them as I think it has to be an individual choice in training vs a competition situation.
I am by no means any type of grappling expert so if anyone says Gringo's idea sucks, then listen to them not me. However when I have a significant weight advantage like you are describing and can't get an RNC. I use the body triangle and try and secure and arm or shoulder limiting an opponents balance and then use my weight advantage and momentum to swing them into either a sitting position or where I have their back and body triangle still but they are on top of me. I find it easier to work attacks from this position (doing some of the less nasty things fatherdog suggested) than from a pure mount. Also when you swing someone over, a lot of times their chin comes up as the natural reaction is to fight for balance.
Once again, this is something that just happens to work for my most of the time and if anyone such a fatherdog, yrkoon, omega, asia, or a few other legitimate instructions/knowledgeable individuals suggest otherwise, listen to them.
If this is with the gi: look for other chokes. Kata ha jime is an excellent one - it works better when you're slightly sideways to your opponent, which is easier with a body triangle than with hooks in.
If he defends the choke, let go of the body triangle and try to overhook the far arm. This leads to a crucifix-like position, and makes reapplying kata ha jime much easier. If that still doesn't work, feed your other leg over his shoulder and go for the triangle.