Just did a bike temporary import + visa run at Mae Sai. All went OK - but it seemed like they really haven't done this much before... Thai side all very helpful - but, unless your Thai is good, I'd take a nice Thai speaker along to help going in and coming back.
You can leave your bike (and pay any overstay fine) at the main Customs building then go to usual Passport Control Out, show them your temp import paper and explain what you plan to do. Someone will hopefully help you do a bit of shuffling around out to the bridge Immigration and Customs desks/kiosks to sort paperwork (you will need 2x copies of your passport and bike registration + one of your current temp import paper, Thai insurance and Thai visa to help when you come back in - in total. Take originals also). Get your paper from Customs saying you've taken your bike out ('out' paper?) then go over (the bridge and over to the other side of the road) to Myanmar Passport Control to pay your 500b, get your passport stamped and get a pass for Tachileik. Park your bike by the side somewhere nearby while you do it. (Myanmar Customs looked like they were thinking whether to stop me, but I just drove on a bit further then walked back. Obviously stop and talk to them if they actually wave you down.) With your pass to show, drive through the rest of the bridge controls (not sure what they are for - but they just waved me through when I showed my pass) into Tachileik.
On return, go back over the bridge, park up by Myanmar Immigration Out and go get your passport. Ride over and cross to park near Thai Passport Control In, walk back around the railings and get your passport stamped - but then don't exit through - explain you have a bike then head back and around the railings to the Thai Immigration and Customs kiosks/desks. Give them your 'out' paper and various copies to get a new temp import paper. Then go fetch your bike and ride back into Thailand.
Like I say, everyone seemed nice and helpful - but not sure how they're going to be if lots of people start doing it and if you go back to do the same thing again and again... Only time will tell - but, taking someone who can talk Thai (if you can't very well), smiling and being polite will certainly help... I think I'll alternate exits between Chiang Khong and Mae Sai too.
http://goo.gl/maps/GJJJ3 Thailand - Main Customs Building. Can park your bike here while you sort stuff and pay any fine due, I guess - though I'd already done this in Chiang Khong.
http://goo.gl/maps/djXWc Thailand Passport Control Out and In - and you can see where the roadside kiosks/desks are on both sides.