It's possible - but things are getting tighter. A little boots-on-the-ground research as of March 2, 2019, found one of the few motorbike rental places in Chiang Mai and Bangkok that will rent a 250cc or bigger motorbike and do the extensive paperwork needed to cross over from Thailand to Laos, at one recommended border crossing, remains the Rider's Corner Restaurant/Bar/Hotel in Chiang Mai.
Brief details:
1) They can arrange for the necessary paperwork (up to 15 pages!) for a customer who rents one of their small fleet at a reasonable cost. Several other rental agencies will no longer allow their rental motorbikes out of Thailand into Laos.
2) A deposit on the motorbike will be required, an amount in cash (no credit cards) and varies on the size and age of the rental motorbike.
Back checking:
The original Green Book (ownership papers) must accompany the motorbike and renter when making the border crossing, so there is a considerable degree of risk of loss to any rental agency if the motorbike ends up in Laos, or Thai Customs or the "No Man's Land" in between due to various reasons.
One tourist recently rented a motorbike from an agency that would not do the Thailand exit and Laos entry paperwork and asked Rider's Corner to do it - a "no go." Rider's Corner can only do the permits/paperwork on motorbikes they own/have Green Book for, so the tourist was politely referred back to their original rental shop.
Reflections:
As things tighten-up on freely roaming around SE Asia on motorbikes, it's refreshing for a wandering spirit to find small businesses, like the Rider's Corner, to step-up and carry the weighty and wavering flag for a bit of solo, unaccompanied adventuring as the larger agencies back away entirely or offer packaged group tours as an attractive all-inclusive option.
It's a very fluid world, that of renting motorbikes and taking them across borders. Often it is the most simple and economic option, other times the barriers and time constraints make it not viable for some travelers. Some call border crossings half of a motorcycle adventure.
This is meant as no detailed thread of information culled from questionable digital sources, just a simple "find" backed by fact checking as of this date. Individual matters such as Visas and vehicle insurance have digital homes elsewhere.
Regards,
Dr. G
Chief of the World Adventure Affairs Desk, CITY BIKE Magazine, researching adventurous affairs on the roads around the globe, recently out of Vietnam, Myanmar, and along the Laos/Thailand border.
Soul Sensual Survivor:
www.greataroundtheworldmotorcycleadventurerally.co m