Google's Chris Wilson announced today that Chrome Canary (the experimental version of Chrome) now contains an initial round of support for the Web MIDI API! Today's build (Version 30.0.1550.2) supports MIDI input on OSX.
So it looks as if browsers are really going to implement the Web MIDI API natively!
Chris also announced an update to his WebMIDIAPIShim glue code for the Jazz plugin -- both of which are currently used by my Assistant Performer
So I have updated the Assistant Performer's code, ready for the time when both the WebMIDIAPIShim and the Jazz plugin become redundant.