Upvote:7
Another option would be to go through Thailand, take a bus from Pakse to Ubon Ratchathani (3 hours leaving 7:30 am).
From there take a bus to Surin running on highway 24, there may be a train (3 hours) as well.
From Surin you can take a swangthaew or minibus to the Cambodian border at Chong Jom (2-3 hours).
16 km south of the border is Along Veng but you may need to take a motorbike from the border.
There are early morning buses from Along Veng to Siem Reap (4 hours on a bad road) but you can also try a shared or private taxi.
Considering you have to cross two borders and change several times without knowing the exact connection times I don't think it feasible to do this in one day either.
Surin seems to be a good enough place to break the trip. They have an Elephant Study centre, some craft villages, temple ruins and the Surin National Museum. More importantly there have several places to spend the night.
I haven't been to any of these places myself, but it sounds rather interesting, it's a bit off the beaten track.