One guy managed to "slip" from Laos into China earlier this year, you can read his remarkable trip reports at www gt-rider.com forums.. there is just one big BUT: he had a chinese-plated bike when he pulled that off. With any other plates it might be a whole different story, I wouldnt count on making it.
If you plan to do China the "official" way, you will, in short, need months of preparation, good contacts and loads of money, and you will be accompanied by a guide. And thats just getting into the country, crossing its entire length might be yet another thing. It doesnt seem to be totally impossible, however, but there are much easier (and probably a lot cheaper!) places to go to.
Some have reported crossing into China "unofficially", with foreign bikes, from Kazakhstan, but thats no guarantee it´ll work from Laos.