I use it in an app and for a limited number of ssh channels (usually 10-20).
But frankly, that code remains before the the CoWork improvements and the arrival of AsyncWork.
Nowadays in most such cases I use the async methods.

I have a new proposal: What if I get rid of queue mechanism and rewrite the package with only blocking mode and optional async transfer methods(using AsyncWork, and naming them agein SFtp::Asyncxxx)?
It won't take more than a week for me to come up with a working SSH package and the existing public API wont change much (only the NB helpers will be gone).
Besides its SC will be lot cleaner.

As to your Get implementation:

I haven't tested this yet, but it shouldn't work in non-blocking mode. (because the execution will be deferred (Get will immediately return) and there is a local variable ("done") )

bool SshChannel::Lock()
{
	if(*lock == 0) {
		LLOG("Channel serialization lock acquired.");
		*lock = ssh->oid;
	}
	return *lock == ssh->oid;
}