The mucous membranes in the vaginal and rectal area allow the gas easier access to blood - and I was thinking that if there was some discomfort because of the esophagus, that it would be an alternative - or even a supplement. There's also a technique called the "bag protocol" - the whole goal is to get as much MMS into the body without causing additional discomfort or herxheimer reaction.
Personally, I find that CDS is easier for me to take than MMS - I'm not sure it's the citric acid, but I tend to have sinus issues with MMS, which leaves me a bit nauseous.