You can fetch the content of any sample as HiseScript Buffer and draw it yourself
This is where I had got up to. But I don't get the playback data from the sampler so I can't draw the playback cursor. Given the performance issues you mentioned I think I will abandon this road :)
My main goal was to be able to update the waveform when the gain level is changed (like in the waveform editor) and have horizontal and vertical zoom.