Well I've found a LOT of potential programs, but so far most of them only record audio, and of the rest all the free ones only work with Windows, or else like Garage Band, do not produce midi files.
I found two hopefuls which seemed to be work-in-progress. I did download both but neither worked.
Others appear to be free at first glance, but it turns out that this is an old reference from the development days, and today (if they're still around) they cost hundreds.
Ah well. I'll just have to sketch in GB, and recreate in PTLE when I get home. Not really a problem.