Can I apply for a Non-O visa without leaving Thailand?

well if that immigration office isn't allowing your husband to apply for a 90 day Non-O based on monthly income there has to be a reason.

It clearly IS a way to meet the proof of funds BUT as I said he'd probably need many many months of international transfers into his thai bank account in his name only for them to consider it.

You on the other hand have no way to get a 90 day Non-O visa inside the country.. Your only option would be for your husband to get his yearly extension, then you go to a thai consulate with marriage certificates etc and get a 90 day Non-O.

OR

You could both go out of the country and try to get Non-O visas. Him by being over 50 and meeting the proof of funds, and you by being married to someone getting a Non-O based on retirement.

What type of visas and/or entry stamps are you on NOW ?

Your husband would have had to been transferring into into a thai bank account in his name only a minimum of 65K baht a month.

IF I was a betting man I'd say they'd want many many months of international transfers into his bank account of the 65k baht to issue the visa in country.

You couldn't get a Non-O visa inside the country period end of story. They are not issued because you're a foreigner married to a foreigner who will get a long term extension.

You can apply for an in country 90 day Non-O visa at the thai immigration office in Chiang Mai IF you have 21 days or more left on either a 30 day visa exempt entry stamp, a 60 day tourist visa entry, OR a 30 day extension from one of them.

You go to the Chiang Mai immigration office and get the hand out that lists the requirements for you to apply for the in country 90 day Non-O visa.

If you don't have 21 days or more left on your current entry stamp you can't get the visa. That's the rule in Chiang Mai.