The Vientiane consulate is supposed to be the easiest to deal with and I was never asked for onward travel proof. Maybe you'll get a second opinion on that.
You can go to Vientiane and apply for a 60 day tourist visa at the consulate there. That can be extended a further 30 days when you are in Thailand. So that should be enough to take you till April. I guess you're not taking that January flight?
Thanks for the tip. But 700 baht is getting close to the cheapest one way air tickets. I'd still use the air option, and no butterflies in the belly at check-in.
I think I'd go with Tod on this one, no offense. I have dealt with some pretty sticky check-in people, some of whom even looked up my ticket in their system. I can see trouble if I showed them a bus ticket with a company they never heard of. In Thailand moreover, air tickets out of the country can be had for as little as 40 dollars so I think that's the safe way to go - consider it the cost of hassle free travel.