Yes, memory streams are supported.
For decoding WavPack files, use the WavpackOpenFileInputEx() function. The callback functions provided by the WavpackStreamReader can get the WavPack data from anywhere (and they don't have to implement "seek" unless you want to be able to seek in the audio playback).
For encoding WavPack files, you again provide a callback function that will accept the completed WavPack blocks and can do with the data whatever it wants.
Hope this is what you were after...