Simply enough in HTTP2: lift out the relevant parts of the response body—turn them into separate resources and refer to them in the original object by their URL. When the client then requests those dependent objects, deliver them uncompressed. (And server-hint/push the dependent objects if you can, of course.)
In practice, there isn't a way I know of to mark the relevant parts of the response body so that the compression, which is usually done as a separate post-processing step, will know to avoid compressing these parts of the stream. And it would be very fragile; forgetting to mark a secret as "do not compress this" would work perfectly fine but reintroduce the vulnerability.