How can I post to ros's callbackQueue()?

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There is no safe option to achieve what you want. Given that a node can be configured with multi-threaded spinner, when you handle the callback of C, there is no guarantee all the callbacks have not been treated already.

A safer solution would be to make your callbacks fill a "normal" C++ structure (like a circular buffer from Boost or a std::queue) and then have your main thread / function making the changes you need.

It will:

1. separate the communication layer from the logic layer
2. make sure your node won't broke the day you will spin with another strategy
3. simplify the algorithm
4. be much more efficient (avoid the serialization/deserialization + the cost of handling the queue). Indeed, another safe option would be to publish from your callback. This probably would work but really seems overkill IMHO.