Not sure exactly what version of Houdini this is supported in, but I don't think this fix was backported to H18.0.
From the upcoming documentation…
When saving an image, husk will process all the attributes on the render product. It looks for settings that begin with driver:parameters: and will pass that data down to the image as a piece of metadata. For example
custom string driver:parameters:OpenEXR:test_string = "Hello world"
custom float driver:parameters:OpenEXR:pi = 3.1415
will add metadata to OpenEXR files named “test_string” and “pi” with the types and values you might expect. Note that OpenEXR doesn't necessarily support all the types of data allowed in USD.
As a note, this should work with any render delegate (not just Karma).