The best way to do is ask the embassy,send them email.Some embassy’s don’t allow but sometimes it is possible if the applicant has a proper visa in Thailand not a “tourist” visa
This agency can help you out to do your visa at thai embassy in laos .The best way is do it outside Thailand.They will assist you throughout the process.Very reliable and trusted.
I would recommend this travel agency that can help you do your visa at thai embassy in Laos.They are reliable and will assist you throughout the process with reasonable price.