nah. roads along the lancang (mekong) in china are mostly nicely paved, even
the rural roads south of jinghong. you'll have trouble finding paved roads along
the mekong in the north of laos...maybe take a freighter from (south of) jinghong
to chiang saen, then follow more or less on the thai side to chiang khong, then take
the slow boat to luang prabang. from there, it's mostly new pavement south to
the tha li crossing into thailand. roads north of the river in laos are mud, paved
in thailand to friendship bridge near vientiane. from there you can follow on
either side to southern laos. regardless, it'll be kinda boring, and you won't
see much river. road is a couple hundred meters to a kilometer or so away
from the water. once in cambodia, it's paved to phnom penh, but you'll have
a really crappy section of gravel/potholes between stung treng and kratie.
don't know about further south.....