Thanks all for the responses,

I now have the Siren MIDI prims (originally written for use with VisualWorks DLLCC) running as an FFI library with Squeak.

The system uses PortMidi, which supports (at least) Mac, Windoze and Linux.

I’ll distribute it as soon as it’s stable and tested, and it’d be great to have it a primitives so that users don’t have to install anything and it’d be faster (less copying).