Upvote:5
The legal answer, as you've already found out, is "no":
4γAre the foreign visitors who completed 72-hour visa-free transit procedures and entered into China permitted to leave Beijing to other Chinese cities?
72-hour visa-free transit foreign visitors are not permitted to leave Beijing to other Chinese cities during their 72-hour visa-free stay. Visitors who have to leave Beijing to other Chinese cities due to force majeure must apply for visas at Beijing Public Security Bureau.
But if you're that desperate to go to the conference, I'd probably go for it anyway. Just don't blame me if you end up sentenced to 10 years of reform-through-labour at Gansu Red Star Toilet Roll Factory #4!
The train is unlikely to be a problem: your passport is checked for identification (= name matches ticket, picture matches face), not visa status, and besides, you'll have a perfectly valid 72-hour transit stamp in there even if they were to check. As you can see, it's very short on detail and doesn't say anything about where you can go:
(courtesy some random Finnish guy via LoyaltyLobby.com)
In the unlikely event that somebody checks and knows you're not supposed to leave Beijing on a transit visa and cares enough to make fuss, you're still in Beijing and you can just plead stupid tourist ignorance.
The problem will be Changchun, and in particular, staying in a hotel overnight, since they will check your passport and visa, and they will likely wonder what's up if you only have a Beijing transit stamp. So either crash 'off the grid' at some local colleague's place if you can (legally they can host you for 24 hours before they need to report your presence to the PSB), find some dodgy room-by-the-hour place that couldn't care less about your ID, or take sleeper trains to/from Changchun.
The absolute worst case scenario would be getting stopped by the PSB in Changchun for a random check and getting busted for being outside Beijing. But short of being in a traffic accident or something this is unlikely, and from anecdotal evidence, I gather this usually results in a "tut tut" and being escorted to the nearest bus/train back to Beijing, not deportation or jail or anything silly like that, especially if you plead the "ignorant tourist" line and wave around your official invitation letter.