I'm assuming that you're referring to ign topic --pub or ign service --req. The rest of the topic/service options should work with custom messages (info, list, echo).
I think this problem can be reduced as: Given a string representation of a Protobuf message (e.g.: example.msgs.StringMsg), can you write a function that creates one of your custom messages and returns a pointer to a google::protobuf::message?
google::protobuf::Message*
CreateMsg(const std::string &_type)
This should be possible to implement if you generate your messages with descriptor information, and then, use the DynamicMessage class from Protobuf. If you can solve this problem first, I'll help you to integrate it with ign.
The way this is solved in Ignition Messages is different. We essentially maintain a map with types and a Protobuf message for each Ignition Message Type. The IGN_REGISTER_STATIC_MSG macro is injected in each message because we use our own message generator to add that code. I don't know exactly where, but the maps is populated inside the Ignition Message shared library.
