A follow up for those looking to do the same in the future:
I ended up recruiting two new friends to join me on the ride just as i left Luang Prabang. We made our way to Luang Nam Tha by bus and found a great deal on some 100cc scooters there (two crap Zongshens and a Honda Wave) for just $4 a day. It would mean a fair amount of backtracking to start and end the ride there but the price of the bikes was better than I'd seen anywhere so we decided to go for it...
For the first day we decided to head down the brand new road that goes to Huay Xai as sort of a test run to see if the bikes would suit us for some extended travels. We went as far as Vieng Phouka and came back - everything was good. We stopped at some villages along the way to ask about nearby caves and waterfalls. A very nice day-trip. Make sure to check out the old broken stupa (forget the name) about 5km from LNT (near the old airstrip) while you're there!
The next day we took off from LNT early, intending to reach Luang Prabang by nightfall. While I'm sure this is easily possible on a "real" bike, we arrived in Udomxai for a late lunch and decided we'd better break it in half and do Luang Prabang the next day. Our stay in Udomxai ended up being a bit wild - we didn't sleep a single minute that night
Don't expect much from this town, though.
So we departed Udomxai a little worse for wear to say the least and ended up taking a wrong turn... 200kms later we're in Pak Beng. Whoops! (Note that the road was excellent from Udomxai to Pak Beng) We made the most of it with a pleasant night there and finally made Luang Prabang the next day after about 9 hours on the road.
For those of you with proper bikes, you might consider doing this route and then taking a short boat ride from Pak Beng to the river town just north of Hongsa (forget the name, sorry) and braving the short but rough ride to Hongsa. From there you could either try the very tough road to Luang Prabang (not in the rain!!) or backtrack to the river and boat it the rest of the way to Luang Prabang.
From Luang Prabang we rode to Muang Sui, spent a night there in some nice bungalows by the lake, and visited the highly disappointing Buddha Cave (Pha Tham) in the morning. We then hit Phonsavan after a nice short ride and visited the jars (#3 is quite nice, very scenic spot, but we scored one flat tire on the janky dirt road which leads there) and opted to continue northward to ______ (completely forgot where we stayed) instead of sleeping in Phonsavan. Another day brought us to Xam Neua which was an excellent ride. Beautiful road. Then we visited Vieng Xai - also fantastic - and took a quick tour to some caves and learned a bit of Laos war history. Rather than stay in Vieng Xai (which would have been quite nice I think) we continued south and then west and got as far as Vieng Thong before ending the day. The roads here got a bit dodgy - I scraped up my foot when i laid down the bike taking a rough corner too fast. Lucky though, no damage and very light injury... There were some short unpaved stretches but nothing that cant be safely done on a small scooter for sure.
From there we rode to Nong Kiaw, spent a quiet evening there and finally returned to Luang Nam Tha via Udomxai the next day.
All in all a fantastic journey. The shitty chinese bikes held up beautifully on the generally good roads and we were happy to have only suffered heavy rain for an hour during the whole ride!
Next time I'm doing it on a XR or Baja though. All I could do was drive past the dozens of inviting dirt tracks we spotted... The GT-Rider map was essential for us. Highly recommended. Even though I speak Lao pretty well, we got so much conflicting information from village locals it was comical. My favorite was one guy who told us it would take 1 hour to drive the remaining 120km to Xam Neua. Get the map
The whole ride took us 9 days, not including the day-trip from LNT. We rode 4-9 hours every day. Note that we would have saved a day if we didn't make the wrong turn in Udomxai!
