Protecting a proprietary protocol

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GPL2 does not require you, or the customer, to make either the source code to the dissector or the executable code of the dissector publicly available.

It does, however:

1. require that anybody who has either the source or executable code be allowed to redistribute it, either as-is or modified, to anybody they wish;
2. require that, if you have provided somebody with the executable code, you make the source code available to them.

At least as I understand it, if, for example, the customer will only be using their dissector in-house, they can keep it in-house (the customer is the recipient of the dissector, not individual employees of the customer).

If, however, the customer wants to provide the dissector to their customers, they have to let the customer give the dissector away, in both source and executable form.

You can get capture data into wireshark through an extcap interface and you can use the dissection engine output from tshark or sharkd. Apart from that there's no pipe interface onto the dissection engine itself to make such a thing possible. This would be non-trivial alone from the amount of state kept by the dissection engine.