I'm not sure you can have any real assurances here because the issue isn't just
the firewall. The real problem is that your dealing with an environment which is
heavily controlled at all layers and the firewall is just one component. It is
quite likely the firewall may simply block connections which it cannot inspect
or has any concern over, but it is probably even more likely that they could
insist clients download and install a government certificate which they could
use in a 'Comodo Style' MItM scenario.
My guess would be that it will depend on whether your connections are actually
noticed and considered to be of any real concern. When you consider the size of
population your dealing with and the amount of data/connections the government
would need to monitor, it is likely that unless something brings what you are
doing tot he attention of the right people, your scheme would be reasonably
secure. However if and when your connections are noticed and there is a desire
to look more closely, then all bets are off.
In an environment where one authority has control over all layers of the
network and can likely insist on clients installing or performing some action
before they are even allowed network access, I think you have little hope of
being able to secure the connection. If the government wants to see it, they
will be able to find a way.
The real question will be down to what the expected level of interest is. Even
the Chinese government has limited resources. At some level a decision has to be
made on what will get the most results for the resources they have. If you were
talking about some service which was going to be used by a large sector of the
population and it was felt that use was going to be detrimental to the ruling
power and that power controls all the infrastructure, you can be pretty certain
they will find a way to access the data. On the other hand, if this is something
only used by a small number of people or if the use is deemed to be of low
threat, then it is likely they would do nothing.
It all really comes down to a risk/benefit analysis from both sides. The real
problem is that one side controls all the client side infrastructure. if they
want in, they will find a way.