How do WWise Source Plugins handle MIDI?

Question

First off this question is a dupe of https://www.audiokinetic.com/qa/1241/recommended-incoming-events-when-designing-source-plugin?show=1241#q1241

However I think I accidentally "answered" that question by responding to it... Anyway, how do WWise source plugins deal with MIDI input? 

I've compiled the sine source plugin myself and debugged it in the authoring tool. If I send a C major chord with three notes, by making the MIDI file's target my synth in the authoring tool, I get three calls to the Init function, and calling in_pSourceFXContext->GetMidiEvent() returns each note-on event in the chord. That's a bit scary - the docs say that the Init function should be where memory allocation occurs, but if I'm trying to play any sort of interesting MIDI pattern, I don't see how this could be feasible for real time playback in a game.

There must be some mistake I'm making, perhaps in the configuration of my plugin... and I can see that Synth One accepts MIDI input as a Frequency Mode parameter, so there must be some way that it's handling this stuff. 

Does anyone have any experience working with MIDI and Wwise, that could perhaps shed light on how plugins are meant to handle more than a single note on event?

Thanks for any help you can provide,

John