Can I obtain a Non-Immigrant O visa based on marriage without having 400,000 THB in my bank account?

Do you have money in Super? Ive used that when I've applied in Australia

Depends where you are living in Thailand a lot of immigration offices will not except the monthly financial requirements only the yearly requirements of 400,000 baht and seasoned correctly. Go and ask your local immigration office what they will except in regards to this.

Yes every entry on that is only for 90 days so it's an extension you are asking about. I don't believe you can get the first extension based on monthly transfers even if they will accept it for subsequent extensions, but they might if you have full 12 months prior to applying. The only way to find out for sure is to pop into the Samui office and ask them.

Are you talking about the 90 day visa or a 12 month extension of stay?

Agent can fix without money or transfers for ~25k

Do an international transfer every calendar month for 12 months, and be able to show a source of foreign income. If minimum qualifications are met, it is easy. If minimum qualifications can be questioned then it becomes difficult.

It might work and it might not. More and more offices are saying IF you don't legally work IN thailand and make the minimum 40K baht a month salary to meet the monthly income requirements based on marriage extension you cannot use monthly incoming transfers from overseas of 40K baht a month for the previous 12 months. It's just a crap shoot on whether your immigration office will or won't allow it, and they wouldn't allow it on the first yearly extension off a 90 day Non-O because you wouldn't have a year's worth of 40K baht a month transfers <- meaning you either legally work here with a work permit and the appropriate minimum salary, your embassy still issues the income affidavit OR you bank the funds.

What country is your passport, you may be able to get the income affidavit from your Embassy?