Can a South African married to a Thai secure a one-year Non-O multiple entry visa from Laos with 400k baht?

There are no verifiable reports that Vientiane gives year-multi-Non-O visa based on marriage even if you have 400K baht in a thai bank account in your name only for 2 months

I would say, take all the documents, updated bank book and GO try it.

With South African passport holders unable to use the thai consulate in Penang Malaysia OR in Savannakhet Lao you don't have a lot of choices..

Dunno if S/A passport is on the list of countries that take 'extended processing' in Hanoi or HCMC, but you could call them and ask

If you go to Vientiane, and they won't give you a year/multi, the worst case is you'll get a 90 day single entry

and

if you have the 400k baht in thai bank for 2 months you could then get the in country extension at the immigration office..

You'd have to contact Pretoria and ask them direct about a year-multi-Non-O based on marriage