From what I understand, following an admittedly very cursory glance around the internet, any negative effects owing to consumption of calcium hydroxide are far overshadowed by the negative effects of the alkaloids themselves, which are known carcinogens. Not meaning to belittle your efforts, or anything, but trying to find an alternative chemical to calcium hydroxide in this situation feels a little bit like telling someone who is fond of snorting cocaine that they should try to use something other than razor blades to form their lines, owing to the risk of cuts on their fingers.
Anyway, on to chemistry. Calcium hydroxide is used widely in the food industry, so it is itself not a toxic substance. Any deleterious effects to mucous membranes would derive from its alkalinity, which is rather mild compared to many alternatives. I don't know a whole lot about the transformative chemical process you are referring to here, and didn't investigate, but any suitable replacement to achieve the same chemical transformations accomplished by calcium hydroxide to the active components in betel nut leaves would probably also have to be alkaline, and so likely you won't be accomplishing that much in the long run by swapping one mild base for another. A milder base would be better, I guess, but whatever transformative process is happening would also probably be less efficient, and thus less desirable to people engaged in this habit.
So, my only partially informed opinion is that you'd be better off focusing your efforts on curbing the betel nut habit itself. I could be glib and say, "good luck with that", but large scale changes to such widespread bad habits can be effected, but usually not through chemistry. Look at what has happened to tobacco use here in the States over the last several decades. That hasn't been primarily due to finding chemical alternatives to tobacco or nicotine, although technology has made cigarettes safer (better filters, etc.)... education efforts, and regulations/taxes, and basic cultural changes (focus on health, etc.) have largely been responsible.
My two cents.